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If* OO/IO BUREAU OF MINES 
 
 IU y^^i* INFORMATION CIRCULAR/1990 
 
 Slip-and-Fall Accidents During 
 Equipment Maintenance in the 
 Surface Mining Industry 
 
 By Thomas J. Albin and W. P. Adams 
 
 ^ ^ \ u.s. BUREAU OF MINES 
 
 o m. J a M r" 
 
 80 
 
 X 
 
 o 
 
 1910-1990 
 
 \ years / THE MINERALS SOURCE 
 
 **au of ^ 
 
Mission: As the Nation's principal conservation 
 agency, the Department of the Interior has respon- 
 sibility for most of our nationally-owned public 
 lands and natural and cultural resources. This 
 includes fostering wise use of our land and water 
 resources, protecting our fish and wildlife, pre- 
 serving the environmental and cultural values of 
 our national parks and historical places, and pro- 
 viding for the enjoyment of life through outdoor 
 recreation. The Department assesses our energy 
 and mineral resources and works to assure that 
 their development is in the best interests of all 
 our people. The Department also promotes the 
 goals of the Take Pride in America campaign by 
 encouraging stewardship and citizen responsibil- 
 ity for the public lands and promoting citizen par- 
 ticipation in their care. The Department also has 
 a major responsibility for American Indian reser- 
 vation communities and for people who live in 
 Island Territories under U.S. Administration. 
 
Information Circular 9249 
 
 * »■ 
 
 Slip-and-Fall Accidents During 
 Equipment Maintenance in the 
 Surface Mining Industry 
 
 By Thomas J. Albin and W. P. Adams 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 Manuel Lujan, Jr., Secretary 
 
 BUREAU OF MINES 
 T S Ary, Director 
 

 O 
 
 o 
 
 Library of Congress Cataloging in Publication Data: 
 
 Albin, Thomas J. 
 
 
 
 
 
 Slip-and-fall accidents during equipment maintenance 
 industry / by Thomas J. Albin and W. P. Adams. 
 
 in the surface 
 
 mining 
 
 p. cm. - (Information circular / 
 
 Bureau of Mines (1988) ; 9249) 
 
 
 Bibliography: p. 10. 
 
 
 
 
 
 Supt. of Docs, no.: I 28.27:9249. 
 
 
 
 
 1. Mine accidents. 2. Mines 
 supplies-Maintenance and repair. 3. 
 Paul), 1961- . II. Title. III. Series: 
 of Mines); 9249. 
 
 and mineral resources-Equipment and 
 Falls (Accidents) I. Adams, W. P. (Wayne 
 Information circular (United States. Bureau 
 
 TN295.U4 [TN311] 
 
 622 s- 
 
 -dc20 [622'.8] 
 
 89-600171 
 
 CIP 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Problem description 2 
 
 Problem background 2 
 
 Objectives 3 
 
 Method 3 
 
 Accident record analysis 3 
 
 Assessment of worker behavior 5 
 
 Mining machine access system analysis 5 
 
 Results 6 
 
 Direct worker behavior 6 
 
 Machine access system design 8 
 
 Workstation design 8 
 
 Discussion 8 
 
 Relative hazard of direct worker behavior and machine access system design 9 
 
 Risk perception 9 
 
 Summary and conclusions 10 
 
 References 10 
 
 Appendix A.-Sample calculations of relative risk ratios 12 
 
 Appendix B.-Calculation of probability of slip-and-fall accident happening to individual during 40-year 
 
 period 13 
 
 ILLUSTRATION 
 
 1. Example of machine access system scoring sheet 7 
 
 TABLES 
 
 1. Operational definitions for antecedent events to slip-and-fall accidents 4 
 
 2. Listing of antecedent events, codes, and frequency noted in accident narratives 6 
 
 3. Antecedent events associated with direct and indirect worker behavior 6 
 
 4. Slip-and-fall accident location, proportion of total work time in location, and relative risk 8 
 
 5. Machine access system ratings 8 
 
 6. Antecedent events related to workstation design 8 
 
 7. Relative risk ratios for direct worker behavior and access system elements 9 
 
SLIP-AND-FALL ACCIDENTS DURING EQUIPMENT MAINTENANCE 
 IN THE SURFACE MINING INDUSTRY 
 
 By Thomas J. Albin 1 and W. P. Adams 2 
 
 ABSTRACT 
 
 This U.S. Bureau of Mines report identifies potential causes of slip-and-fall accidents occurring during 
 surface mine equipment maintenance and describes the relative roles of direct worker behavior and 
 machine design. The relative roles of human behavior and machine design in the causation of slip-and- 
 fall accidents were determined through analysis of accident records, observations of maintenance worker 
 behavior, and evaluation of mine machinery. From these data, relative risk ratios were calculated. 
 Behavior had a relative risk ratio of 1.5; machine design, specifically access systems design, had a relative 
 risk ratio of 2.2. Of the access systems, ladders had the highest relative risk ratio, 7.0. This study 
 suggests that accident intervention would be most profitably made in improving the design of systems 
 used to access maintenance worker areas of mining equipment. 
 
 'industrial engineer, Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN (now with 3M Company, St. Paul, MN). 
 2 Mechanical engineer, Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN (now with University of Nebraska-Lincoln, 
 Lincoln, NB). 
 
INTRODUCTION 
 
 PROBLEM DESCRIPTION 
 
 Recent research conducted by the U.S. Bureau of 
 Mines has shown that accidents occurring during main- 
 tenance of mining equipment constituted 34 pet of all 
 surface mining accidents from 1978 to 1984 (i). 3 Does this 
 mean that maintenance is more hazardous than other min- 
 ing activity? During 1986, the only year for which demo- 
 graphic data regarding miners are available, there were an 
 estimated 27,864 maintenance workers in the surface 
 mining industry (2-3). Although these workers accounted 
 for only 6.7 pet of all hours worked, they were involved in 
 16.8 pet of all surface mining accidents (4). Based on 
 these estimates, it appears that maintenance is 2-1/2 times 
 more hazardous than other surface mining activities. 
 
 Contemporary mining equipment is quite large, and 
 maintenance workers must move about on it to perform 
 their jobs. A large part of their work is performed in the 
 presence of fluids, fuels, and lubricants. Dusts, ore 
 particles, and other associated debris may also degrade 
 surface footing properties. All of these substances put 
 maintenance workers at risk for slips and falls. Bureau 
 research found that slip-and-fall accidents constitute 
 approximately 20 pet of all surface mining maintenance 
 accidents. 
 
 Slip-and-fall accidents, as a group, are more severe than 
 are other mining accidents. A standard definition of 
 accident severity, as used by the U.S. Mine Safety and 
 Health Administration (MSHA), is the sum of the lost 
 workdays plus the statutory number of penalty days 
 assessed to each accident plus one-half the restricted duty 
 workdays. One study found the average accident severity 
 for all surface mining accidents to be 18.5 days compared 
 with 33 days for slip-and-fall accidents (5). Thus slips and 
 falls, as 20 pet of all maintenance accidents with an 
 average severity 1.8 times the average, account for 36 pet 
 of all maintenance accident severity. 
 
 Slips and falls are clearly an important safety problem 
 that costs individuals and mine operating companies 
 dearly. The individual endures both the economic and 
 physical costs of the accident, while the operating company 
 endures increased insurance costs and losses in produc- 
 tivity. It has been estimated that the average direct cost 
 per case of all surface mining accidents is $14,000 (<5). If 
 the relationship between accident severity and accident 
 cost is linear, then an average slip-and-fall accident has an 
 associated cost 1.8 times the overall average, or $25,200. 
 This estimate is based on (1) loss in personal income, (2) 
 compensation of wages, (3) death and disability benefits, 
 (4) medical costs, (5) postaccident losses as a result of 
 fatality or amputation, and (6) investigation of a fatal 
 accident (6). The estimate does not include the costs 
 of (1) loss of life, (2) fines, (3) lawsuits, (4) loss of 
 
 Italic numbers in parentheses refer to items in the list of references 
 preceding the appendix at the end of this report. 
 
 equipment, (5) productivity losses due to workplace dis- 
 ruption, (6) productivity losses due to temporary or 
 permanent shutdowns, (7) productivity losses due to 
 inexperienced replacement workers, or (8) long-term 
 followup medical or rehabilitation treatment (6). 
 
 The expected loss to a mining operator in a given time 
 period can be estimated. During the first 6 months of 
 1987, individuals employed in all sectors of surface mining 
 operations, including coal, metal-nonmetal, and sand and 
 gravel, with their associated plants and mills, worked a 
 total of 398,179,605 h (7). During the same time period, 
 1,163 slip-and-fall accidents occurred in these same 
 operations (7). The expected incidence rate of slip- and- fall 
 accidents per any given unit of time may be obtained from 
 these data by division. It is the usual practice to base the 
 incidence rate on the number of accidents per 100 work 
 years, where a work year is defined as 2,000 h worked (one 
 full-time equivalent) (7). Using this method, the expected 
 incidence rate of slip-and-fall accidents in surface mining 
 is 0.58 incidents per year per 100 full-time equivalents. 
 (An alternative method for estimating incidence rates is 
 described in reference 8.) The minimal expected cost of 
 a slip-and-fall accident per 100 full-time equivalents is the 
 product of the probability of a slip-and-fall accident (0.58), 
 times the linearly estimated cost of a slip-and-fall accident 
 ($25,200), or $14,616. Thus, each surface mining operation 
 should expect to incur approximately $15,000 in costs be- 
 cause of slip-and-fall accidents per 100 full-time employees 
 annually. 
 
 This $15,000 cost figure can be used to "bench mark" 
 intervention efforts aimed at preventing or decreasing the 
 rate of slip-and-fall accidents. As an example, assume a 
 mine with 100 full-time employees is contemplating spend- 
 ing $20,000 in permanent improvements that would de- 
 crease slip-and-fall accidents by 50 pet. This is an initial 
 investment of $20,000 and a reduced expected annual cost 
 of $7,500 (15,000 x 0.5). The expected payback period is 
 2.67 years, calculated by dividing $20,000 by $7,500 per 
 year (9). 
 
 However, this example assumes that the miner has 
 some knowledge of the causes of slip-and-fall accidents 
 prior to addressing them, knowledge of the effect on the 
 slip-and-fall incidence rate of remediating them once they 
 are identified, and an idea of the cost of implementing the 
 remediative measures. While the last might be estimated 
 with accuracy, the first two are less well studied in surface 
 mining. 
 
 PROBLEM BACKGROUND 
 
 Remediation of the causes of slip-and-fall accidents re- 
 quires knowledge of the kinds of causes and their nature. 
 In a general discussion of accident safety, two diametrically 
 opposed explanations of accident causation are given. The 
 first of these is that "stupid, careless, negligent people are 
 
the cause of 90 pet of all accidents and that the tools, 
 machinery, or processes should not even be considered as 
 being primary potential causes" (11). The other extreme 
 explanation blames poorly designed products as the cause 
 of accidents. The belief is that products should be so 
 designed that, even in the presence of untrained careless 
 persons, the product will have human fail-safe procedures 
 features that will provide protection for persons from their 
 own errors and negligence (10). While these two oppo- 
 sites are recognized as extremes, they serve a heuristic 
 purpose: To what extent are individual behavior and 
 machine design involved as risk factors in slip-and-fall 
 accidents in surface mining maintenance? 
 
 A study done by MSHA in 1985 analyzing ladder falls 
 from off-road haul trucks stated that "safe ladder systems 
 are needed to prevent men from falling; however, in- 
 sufficient training and unsafe work practices appear to be 
 major causes of injuries associated with haul truck ladder 
 systems" (11). It is worth noting that the researchers were 
 unable to classify 65 pet of the accidents as to probable 
 cause. Unsafe work habits were defined in this study as 
 (1) missing a step, (2) carrying articles, (3) jumping from 
 the ladder, (4) catching clothing or ring on a step or 
 handrail, (5) not using the handrail, (6) falling from a 
 moving truck and, (7) facing away from the ladder. While 
 the classification of some of these behaviors might be 
 debated, the import remains, individuals engage in risky 
 behavior in the job environment. 
 
 Risk may be defined logically as the expected loss re- 
 sultant from a chosen alternative behavior (12), and it may 
 be further divided into objective and subjective risk that 
 often differ (13). Objective risk is the verbally expressed 
 level of risk, i.e., "it is unsafe to climb a ladder while 
 carrying an object in my hands," versus the subjective risk 
 assessment, i.e., "I've carried this part up this way a 
 hundred times before without any problem." Behavior 
 clearly plays a role in slip-and-fall accidents, and risk 
 perception logically plays an important role in behavior. 
 
 Previous work by the Bureau on slip-and-fall accidents 
 during maintenance has focused on the design of access 
 systems (such as ladders, steps and stairs, and walkways) 
 for mining machines (14). Serious design problems, 
 particularly the height of the first step off the ground, were 
 identified and remediative devices were developed. 
 
 Risk behaviors associated with slip-and-fall accidents 
 are, currently, not well described. Some are well known, 
 i.e., descending a ladder facing outwards, but there are 
 likely others as yet unidentified. In addition, the risk 
 associated with these behaviors has not been directly 
 assessed. Similarly, machine design problems have typi- 
 cally been ascribed to existing access systems, i.e., the 
 height of the first ladder step above the ground, but it is 
 not clear if other design problems exist. 
 
 It is apparent that remediation of slip-and-fall accidents 
 will be necessary. It is also apparent that both behavior, 
 including risk perception, and machine design are probable 
 causes of such accidents, but that their relative importance 
 is unclear. It is important to understand their relative 
 causal roles in order to develop effective remediation 
 strategies that will prevent or decrease slip-and-fall 
 accidents. 
 
 OBJECTIVES 
 
 This research is part of the Bureau's program to en- 
 hance the health and safety of mine maintenance workers. 
 There are two major objectives of this study. The first is 
 to develop a description of antecedent events to slip-and- 
 fall accidents that occur during the maintenance of surface 
 mining equipment. The second is to determine the relative 
 roles of behavior and machine design in slip-and-fall ac- 
 cident causation. These objectives will be accomplished 
 through analysis of accident record narratives and field 
 data. 
 
 The description of behavior included identification and 
 description of specific risk behaviors, estimation of the 
 proportion of all behavior that the identified risk behaviors 
 constitute, and finally, empirical estimates of risk resultant 
 to the behaviors. 
 
 Machine design was assessed both by compliance of ac- 
 cess system design with the guidelines for access systems 
 of off-road equipment described in the Society of Automo- 
 tive Engineers Standard SAE J 185 (15) and by the evalua- 
 tion of sufficiency and adequacy of existing access systems 
 as determined through the analysis of accident records. 
 Empirical estimates of the relative risk of various access 
 system elements were developed. 
 
 METHOD 
 
 ACCIDENT RECORD ANALYSIS 
 
 Accident records, including a short narrative description 
 of the accident, were obtained from the MSHA data base 
 through use of the Bureau's Accident Data Analysis 
 (ADA) program. These accident records were drawn from 
 the surface mining industry, which includes coal, metal and 
 nonmetal, and aggregate mines. All slip-and-fall accidents 
 involving surface mine maintenance workers in the years 
 1985, 1986, and the first half of 1987 were analyzed. These 
 
 were the most recent data available at the time of compila- 
 tion. A total of 1,381 accident records were obtained. 
 Based on 1986 estimates (2-3), these records pertain to 
 18,362 mines and 27,864 workers. 
 
 The accident narratives were typically not prepared by 
 the injured individual. They were most commonly prepar- 
 ed by safety officers, next followed in frequency by 
 foremen, and finally by the injured individuals. Some addi- 
 tional caveats regarding the narratives should be noted. 
 The narratives were prepared after the fact. As such, their 
 
accuracy and validity may be fairly questioned. First, it is 
 conceivable, and even likely, that important causal factors 
 have not been included in some of the narratives. Second, 
 the reliability of the narratives may be questioned. Given 
 the free-form nature of the narrative, an individual might 
 note and report widely differing narratives of highly similar 
 events. This problem is further complicated when many 
 different individuals are reporting accidents. Nonetheless, 
 these records do contain information that has an important 
 heuristic value for future research. 
 
 The narrative records were read and classified by a 
 single rater (Adams) using a prepared list of operationally 
 
 defined event codes. This rater is a graduate engineer 
 with approximately 1-year's experience in human factors. 
 In order to assess rater reliability, 100 accident narratives 
 were randomly drawn and classified by a second rater 
 ( Albin). The codes for these 100 records were compared 
 with the codes prepared by Adams. A coefficient of agree- 
 ment (7(5) of 0.93 (p < O.'OOl) was obtained. 
 
 Event codes and definitions were created for any events 
 discovered in the course of analysis that did not fit events 
 defined in the original list. The final list of events and 
 their definitions are presented in table 1. 
 
 Table 1 .-Operational definitions for antecedent events to slip-and-fall accidents 
 
 Event Definition 
 
 Ladder structural failure Ladcer component failed while in use. 
 
 Slipped on ladder Individual slipped while on ladder. 
 
 Ladder slipped or fell Portable ladder slipped or fell while in use. 
 
 Handrail not used Individual did not use handrail while climbing ladder, stairs, etc. although hands 
 
 were free. 
 Carrying object while using ladder . . Individual carried object while climbing or descending ladder, so that hands 
 
 were not free to grasp handrail. 
 
 Handrail did not arrest Handrail friction was insufficient or excessive load on hand gripping rail. 
 
 Fell from ladder Individual fell from ladder. 
 
 Stepped off access system Individual slipped and fell while stepping from ladder, step, etc. to base 
 
 surface. 
 Siipped on surface Individual slipped on mud. ice. etc. while walking on parking lot, pitfloor, etc. (not 
 
 incasing building floors). 
 Unmarked hazard Unmarked or unguarded hazard, particularly holes in surface made by missing 
 
 plates of deck grating. 
 
 Tripped on uneven surface Indivic-al tripped on projection from surface. 
 
 Tripped on grating Individual tripped when cleats of work boots were caught in grating. 
 
 Siipped on debris Individual slipped on debris, scrap, ore. tools, etc. on surface. 
 
 Siipped on spill Individual slipped on spilled substance, such as lubricant, coolant, water. 
 
 Slipped on work surface Individual slipped on surface of work area (including building floors). 
 
 Slipped on stairs or step Individual slipped or fell on stairs or step. 
 
 Portable stairs moved Portable stairs moved while in use. 
 
 Siipped off machine Individual slipped or fell from machine because of poor surface traction. 
 
 Slipped on machine Individual slipped while on mobile or stationary machine and fell on machine. 
 
 Slipped on machine access system . Individual slipped and fell while stepping onto ladder, step. etc. from base 
 
 surface. 
 
 Slipped on catwalk Individual slipped on walkway or catwaJk. 
 
 Slipped on deck Individual slipped on deck of mobile or stationary machine. 
 
 No escape route Escape route was not designed into system. 
 
 Jumped Individual jumped from one location to another, as in jumping between 
 
 machines or down from machines. 
 Inadequate workstand Workstand did not have sufficient surface traction or was lacking a necessary 
 
 component, such as a guardrail. 
 No guardrail Individual fell off object, such as workstand or platform because guardrail was 
 
 not in place. 
 
 Guardrail structural failure Guardrail failed because of load. 
 
 No toe rail Individual's foot slipped over edge or object, such as tool, dropped off because 
 
 toe rail was not provided. 
 Structural failure Failure of structural member not part of access system. Failure of structural 
 
 member not designed for use as access system but being used for such is 
 
 coded as inadequate workstand. 
 
 Inadequate access design Access system not designed properly to enable adequate access to machine. 
 
 Unexpected energy release Individual fell when unexpected energy was released, such as wrench slipping 
 
 or rope breaking. 
 Work boots not cleaned Individual slipped and fell because of decreased traction due to mud, oil, etc. 
 
 on boots. 
 
 Clothes caught on object Individual slipped and fell when clothes caught on object. 
 
 Unknown Not enough information to classify. 
 
 Access system not used Individual did not use access system although one was available. 
 
 Knee gave way Individual's knee buckled causing slip and fall. 
 
 Falling or raising object Accident caused by falling object, or while individual was raising object. 
 
 Bumped head Accident caused by bump to head. 
 
 Carried object Individual was carrying object in hands prior to accident. 
 
 Coce 
 
 10 
 
 11 
 
 12 
 
 13 
 
 14 
 
 15 
 
 16 
 
 17. 33. 41 
 
 20 . 
 
 21 
 
 22 
 
 23 
 
 24 
 
 25 
 
 26 
 
 30. 32 . . 
 
 31 
 
 40 
 
 42 
 
 43 
 
 44 
 
 45 
 
 46 
 
 47 
 
 50 
 
 51 
 
 52 
 
 53 
 
 54. 64 . . 
 
 55 
 
 60 
 
 61 
 
 63 
 
 65 
 
 66 
 
 70 
 
 71 
 
 72 
 
 73 
 
ASSESSMENT OF WORKER BEHAVIOR 
 
 The analysis of accident narrative reports described in 
 the section "Accident Record Analysis" was used to de- 
 velop a list of direct worker behaviors that were antece- 
 dent to slip-and-fall accidents. These behaviors include 
 using a ladder while carrying an object in the hands; 
 jumping from a machine; not using an access system when 
 one is available; and not cleaning oil, mud, etc. off of work 
 boots. 
 
 For the field phase of the study, two researchers 
 observed maintenance work activity for a combined 80 h 
 of observation time. During this time, four maintenance 
 workers were scheduled to be filmed while performing 
 their normal duties for 2 h each on 2 separate days over a 
 period of 4 days. Filming was done while maintenance 
 personnel accessed front-end loaders, bulldozers, off-road 
 haul trucks, road graders, power shovels, and various 
 stationary machines such as crushers, etc. In actuality, 
 much of the maintenance work observed during these 
 filming periods took place in areas that were physically 
 inaccessible to the camera. Consequently, only about 2 h 
 film was gathered on each of the four individuals. One 
 individual at a time was filmed, starting on the second and 
 sixth hours of each shift in order to control for time 
 effects. Both the days when filming occurred and the 
 order of filming during the shift were randomly deter- 
 mined for each individual. While the possibility of a 
 Hawthorne effect exists, previous research has suggested 
 that such behavior changes are transient and disappear 
 after repeated observations (17). 
 
 Behavioral samples were obtained by viewing randomly 
 determined portions of 1 day's videotape of each of the 
 four maintenance workers while they serviced mining 
 equipment. The days used were randomly selected for 
 each individual as follows. Each of the four selected tapes 
 was numbered in sequence. The lengths of the selected 
 videotapes, as indicated by the tape player's incremental 
 counter, were added, in order of sequence, to generate 
 a total tape length. A BASICA program was written to 
 generate a list of 100 random numbers ranging between 
 and 1. Each of these numbers was then multiplied by 
 the total tape length to generate a point at which the ob- 
 servation was to occur. The tape was then viewed at these 
 points. A tally was made of all instances where one of the 
 high-risk behaviors was observed. 
 
 During each day that an individual maintenance worker 
 was selected for filming, that individual was under continu- 
 ous observation by an observer. The observer recorded 
 information on the relative proportions of time that 
 maintenance workers spent in various locations, including 
 various access systems. These locations were (1) on a 
 ladder, (2) on the ground, (3) on stairs, (4) stationary on 
 a machine, (5) on a workstand, (6) climbing on a machine 
 
 where no prepared access existed, (7) on a walkway, 
 (8) operating a machine, and (9) miscellaneous. 
 
 These worker location data were collected continuously 
 on all four maintenance workers over the 4-day period 
 using a systematic random sampling procedure (18) at 
 10-min intervals after a randomly determined starting 
 point. These data were recorded in a notebook. 
 
 The objective of this phase of the study was to estimate 
 two proportions: the proportion of high-risk behaviors and 
 the proportion of time spent in various locations. The 
 estimates of these proportions were accomplished via a 
 work-sampling technique (18). This method may be used 
 to estimate the precision of the estimate of a proportion 
 using equation 1: 
 
 N = (p' (1 - pO^/D 2 , 
 
 (1) 
 
 where N 
 P' 
 
 and 
 
 D 
 
 = number of samples required, 
 
 = first-guess estimate of the proportion,' 
 
 = z-score for any desired confidence 
 interval, 
 
 = desired precision of the estimate of the 
 proportion, expressed as a decimal frac- 
 tion and interpreted as the proportion 
 plus or minus D pet. 
 
 While z was specified prior to the study as 1.96, and 
 time constraints suggested a maximum sample size, N, of 
 100, the proportion of risk behaviors was unknown at the 
 beginning of the study, resulting in an unknown degree 
 of precision. It was decided to use a conservative pro- 
 cedure (18) to estimate the precision for a sample size 
 of 100 observations. This procedure consists of setting p' 
 equal to 0.5 and solving equation 1 for D. Using this pro- 
 cedure yielded a value of 0.09 for D. This was considered 
 to be an acceptable level of precision, particularly as 
 the true proportion was considered likely to be much less 
 than 0.5, which would result in increased precision of the 
 estimate with a sample size of 100. 
 
 MINING MACHINE ACCESS SYSTEM ANALYSIS 
 
 Access systems of mobile mine equipment were 
 evaluated for new (1988) and older (pre- 1988) machinery. 
 Access systems of the new machines were evaluated with 
 the manufacturers' permission and cooperation at the 1988 
 MINEXPO show in Chicago, IL. Of the new off-road 
 machines examined, three were front-end loaders and two 
 were trucks. These machines are a reasonable representa- 
 tion of equipment that would be found in a surface mine, 
 with the notable exception of power shovels. 
 
Of the older machines evaluated at a cooperating min- 
 ing operation, two were front-end loaders and two were 
 off-road haul trucks. All machines were evaluated for 
 compliance with the recommended access system design 
 guidelines published by SAE (15). Scoring sheets were 
 constructed for this evaluation, which depicted different 
 access system components and the relevant dimension lines 
 of each. Each relevant dimension of the access system, 
 e.g., ladder width, was given a letter code. The evaluator 
 recorded the site of any particular access system 
 component on the machine, e.g., cab access, and the 
 measurements appropriate to that particular access system 
 element by letter code on the sheet. A sample scoring 
 sheet is shown in figure 1. 
 
 Dimensions for each access system element were scored 
 to indicate compliance with the SAE J185 standard. The 
 SAE J185 standard gives minimum, maximum, and rec- 
 ommended dimensions for each access system element. 
 If the access system of a machine was within 10 pet, 
 plus or minus, of the recommended dimension, it was 
 given a score of 1. An access system element whose 
 dimensions were between the maximum and minimum 
 given in SAE J185 but not within a 10-pct range of the 
 recommended dimension was given a score of 2. An 
 access system element whose dimensions were outside the 
 maximum or minimum values was given a score of 3. 
 Each access system component was given a composite 
 score, which was the arithmetic average of its subelements' 
 scores, i.e., the score for a ladder would be the average of 
 the score for step width, handrail diameter, etc. 
 
 RESULTS 
 
 A complete listing of antecedent events, their associated 
 codes, and the frequency with which they were noted in 
 the accident records is presented in table 2. The total 
 number of events exceeds the number of accidents because 
 some accidents had more than one antecedent event noted. 
 
 Direct Worker Behavior 
 
 Direct volitional behavior of the individual involved in an 
 accident was noted in 8.9 pet of all accident narratives. 
 These behaviors included descending a ladder facing 
 outward; carrying an object while using a ladder; jumping 
 from a machine; not cleaning mud, oil, etc. from work 
 boots; or not using an access system when one is available. 
 
 Using the random sampling procedure described in the 
 "Methods" section, 100 observations were made of main- 
 tenance workers' behavior while they were maintaining 
 mining equipment. During these observations, six in- 
 stances of direct, high-risk behaviors, such as jumping off 
 the machine, were observed. Thus, the estimate of the 
 proportion of high-risk behaviors, as calculated from 
 equation 1, is 0.06, plus or minus 0.05. A summary of all 
 behavioral data from both the accident records and field 
 observations is presented in table 3. 
 
 Table 3 indicates that an additional 237 accidents (17.2 
 pet) had a possible behavioral component, although the 
 
 behavior was not necessarily that of the injured individual. 
 These events were unmarked hazards, such as removed 
 floor plates, debris on surface, and spills on surface. 
 
 Table 2.-Listing of antecedent events, codes, and 
 frequency noted in accident narratives 
 
 Event Code Frequency 
 
 Slipped on surface 20 170 
 
 Slipped off machine 40 170 
 
 Unknown 65 141 
 
 Slipped on spill 25 129 
 
 Stepped off machine 41 116 
 
 Tripped on uneven surface 22 104 
 
 Unexpected energy release 60 97 
 
 Slipped on machine 42 90 
 
 Inadequate workstand 50 89 
 
 Slipped on work surface 26 82 
 
 Slipped on debris 24 73 
 
 Carried object 73 62 
 
 Slipped on step 32 55 
 
 Slipped on stairs 30 53 
 
 Jumped 47 51 
 
 Fell from ladder 16 50 
 
 Slipped on ladder 11 48 
 
 Stepped off structure or equipment other 
 
 than machine 33 39 
 
 Ladder slipped or fell 12 38 
 
 Access system not used 66 38 
 
 Unmarked hazard 21 35 
 
 Falling or raising object 71 34 
 
 Other structural failure 54 32 
 
 Knee gave way 70 27 
 
 Work boots not cleaned 61 25 
 
 Stepped on machine access system .... 43 22 
 
 Stepped off ladder 17 12 
 
 Slipped on catwalk 44 12 
 
 Slipped on deck 45 9 
 
 No guardrail 51 9 
 
 Carrying object while using ladder 14 7 
 
 Tripped on grating 23 6 
 
 Portable stairs moved 31 6 
 
 Guardrail structural failure 52 6 
 
 Clothes caught on object 63 6 
 
 Ladder structural failure 10 5 
 
 Inadequate access design 55 4 
 
 Bumped head 72 3 
 
 Handrail not used 13 2 
 
 Handrail did not arrest 15 2 
 
 No escape route 46 2 
 
 No toe rail 53 2 
 
 Total structural failure 64 1 
 
 Table 3.-Antecedent events associated with direct 
 and indirect worker behavior 
 
 Event Code Frequency 
 
 Direct: 
 
 Jumped 47 51 
 
 Access system not used 66 38 
 
 Work boots not cleaned 61 25 
 
 Carried object while using ladder 14 7 
 
 Handrail not used 13 2 
 
 Total 123 
 
 Indirect: 
 
 Slipped on spill 25 129 
 
 Slipped on debris 24 73 
 
 Unmarked hazard 21 _35 
 
 Total 237 
 
Model : 
 
 Loc 
 
 Loc 
 
 Loc 
 
 Loc: 
 
 Loc 
 
 Loc 
 
 Loc 
 
 A 
 
 A . 
 
 A 
 
 A 
 
 A 
 
 A 
 
 A 
 
 R 
 
 R 
 
 R 
 
 R 
 
 R 
 
 R 
 
 R 
 
 C. 
 
 r. 
 
 r. 
 
 c 
 
 r. 
 
 C 
 
 C 
 
 n 
 
 n 
 
 n 
 
 D 
 
 n 
 
 
 
 
 
 F 
 
 v. 
 
 F 
 
 E 
 
 E 
 
 F, 
 
 F 
 
 Figure 1. -Example of machine access system scoring sheet (Loc = location). (Courtesy Society of Automotive Engineers (15)) 
 
Machine Access System Design 
 
 Table 4.-Slip-and-fall accident location, proportion of 
 total work time in location, and relative risk 
 
 The accident narratives indicate that access system 
 elements (ladders, steps and stairs, and walkways) were the 
 location of 291 accidents (21.1 pet of total). Field data 
 indicate that workers spend 10.8 pet of their total work 
 time on ladders, steps and stairs, and walkways. A 
 convenient way of evaluating the relative risk of access 
 system elements is to divide the percentage total of all 
 accidents that occur on an access system element by the 
 percentage of total work time during which the worker is 
 located on that element. The higher the obtained ratio, 
 the more hazardous the access system element. Such an 
 analysis is presented in table 4. This table incorporates the 
 accident narrative data with the data obtained in the field 
 regarding the location of the worker. Ladders, with a ratio 
 of 6.1, and steps and stairs, with a ratio of 2.4, are the 
 most hazardous of the access system elements. The cate- 
 gories of surfaces and machines have been added to this 
 table for completeness. 
 
 Measurements were taken on the access systems of 
 eight new machines at the 1988 MINEXPO in Chicago, 
 IL. Three of these machines were front-end loaders and 
 two were off-road haul trucks. Similar measurements of 
 older machines were taken at a cooperating mining 
 operation on two front-end loaders and two off-road haul 
 trucks. These data are presented in table 5. There is a 
 noticeable improvement in the design of the newer 
 equipment access systems. The overall average rating for 
 new machine access systems was 1.7 as compared with 2.0 
 for older machines. Older truck ladders deviate the most 
 from the SAE J 185 guidelines with an average of 2.6. 
 
 Workstation Design 
 
 Maintenance workers often gain access to the workplace 
 through the use of workstands or other means of access 
 that are not an integral part of the mining equipment. 
 A common example is a stepladder. A number of such 
 accidents where the workstand design appeared to be at 
 fault were noted in the accident narratives. These 
 incidents were grouped with other records where there 
 were apparent design problems with the workstation. 
 Three hundred instances of workstation design problems 
 were noted in the accident narratives. These data are 
 presented in table 6. 
 
 Location Frequency 
 
 Pet total 
 accidents 
 
 Pet total 
 work time 
 
 Ratio 
 
 Access systems: 
 
 Ladders 
 
 Stairs and steps 
 Walkways .... 
 
 Surfaces 
 
 Machines and all 
 else 
 
 160 
 
 114 
 
 12 
 
 252 
 
 843 
 
 11.6 
 
 8.3 
 
 .9 
 
 18.2 
 
 61.0 
 
 1.9 
 
 3.5 
 
 5.4 
 
 38.9 
 
 51.0 
 
 6.1 
 
 2.4 
 .17 
 .47 
 
 1.20 
 
 Table 5.-Machine i 
 
 access system ratings 
 
 
 Machine 
 
 Ladders 
 
 Walkways 
 
 Handrails 
 
 Average 
 
 NEW 
 
 Front-end loaders: 
 
 A 
 
 B 
 
 C 
 
 2 
 
 2.1 
 
 1.5 
 
 1 
 2 
 1 
 
 1.8 
 2.7 
 1.5 
 
 1.6 
 2.3 
 1.4 
 
 Average 
 
 1.9 
 
 1.3 
 
 2.0 
 
 1.8 
 
 Trucks: 
 
 D 
 
 E 
 
 1.5 
 1.8 
 
 1.3 
 1.5 
 
 1.7 
 1.5 
 
 1.5 
 1.6 
 
 Average 
 
 1.7 
 
 1.4 
 
 1.6 
 
 1.6 
 
 Overall average 
 
 1.8 
 
 1.4 
 
 1.8 
 
 1.7 
 
 OLDER 
 
 Front-end loaders: 
 
 F 
 
 G 
 
 1.6 
 1.8 
 
 2 
 2 
 
 NA 
 1.9 
 
 1.8 
 1.9 
 
 Average 
 
 1.7 
 
 2 
 
 1.9 
 
 1.9 
 
 Trucks: 
 
 H 
 
 I 
 
 2.5 
 2.7 
 
 1.7 
 1 
 
 2.7 
 2 
 
 2.3 
 
 1.9 
 
 
 
 Average 
 
 2.6 
 
 1.4 
 
 2.4 
 
 2.1 
 
 Overall average 
 
 2.2 
 
 1.7 
 
 2.2 
 
 2.0 
 
 NA Not available. 
 
 Table 6.-Antecedent events related to workstation design 
 
 Event Code Frequency 
 
 Tripped on uneven surface 22 104 
 
 Inadequate workstand 50 89 
 
 Ladder slipped or fell 12 38 
 
 Other structural failure 54 32 
 
 No guardrail 51 9 
 
 Guardrail structural failure 52 6 
 
 Portable stairs moved 31 6 
 
 Ladder structural failure 10 5 
 
 Inadequate access design 55 4 
 
 Handrail did not arrest 15 2 
 
 No toe rail 53 2 
 
 Total structural failure 64 1 
 
 Total 298 
 
 DISCUSSION 
 
 This study sought to identify antecedent events to slip 
 and fall accidents in order to develop a better under- 
 standing of the causation of such accidents. In addition, it 
 sought to describe the relative roles of worker behavior 
 and machine design contributory to accidents. It was 
 further hypothesized that the perception of risk would 
 
 affect behavior. Data were collected from two different 
 sources: reports of surface mining maintenance accidents 
 and a field study of maintenance workers. 
 
 There were three major findings in this study. First, 
 analysis of the accident narratives showed that 286 acci- 
 dents, or 20.7 pet of the total number of accidents studied, 
 
occurred on access system elements such as ladders, stairs 
 and steps, and walkways. In contrast, workers spent only 
 10.8 pet of their total work time on access system ele- 
 ments. Second, a group of direct worker behaviors ante- 
 cedent to slip-and-fall accidents was identified in the 
 accident narratives. These direct behaviors were found in 
 123 accident narratives, or 8.9 pet of the total number of 
 accident reports. Field studies indicated that these 
 behaviors constituted approximately 6 pet of all worker 
 behaviors, plus or minus 5 pet. Third, older machines 
 were ranked lower than new machines when measured 
 against the SAE J 185 standards, with overall rankings of 
 2.0 and 1.7, respectively. Ladders and handrails on older 
 off-road haul trucks were particularly problematic. 
 
 Finally, during the 80 h of observation, a single slipping 
 accident was observed. This slip did not result in an injury 
 and occurred on a fixed ladder. No direct behavior ante- 
 cedents were observed prior to this incident. 
 
 RELATIVE HAZARD OF DIRECT WORKER 
 
 BEHAVIOR AND MACHINE ACCESS 
 
 SYSTEM DESIGN 
 
 In order to discuss the relative roles of direct worker 
 behavior and machine design in the causation of main- 
 tenance slip-and-fall accidents, a mutual basis of com- 
 parison between these two classes of antecedent events 
 must be established. The method used in this report is to 
 calculate relative risk ratios. Relative risk ratios compare 
 the rates of occurrence of an accident in one location or 
 category with the rate of occurrence of similar accidents 
 elsewhere. A relative risk ratio greater than 1 indicates a 
 higher degree of risk associated with that given category. 
 The general formula for relative risk ratios is given in 
 equation 2 (19). 
 
 RR = 
 
 (AcC/hC) 
 
 (TAc - AcC)/Th - hC) 
 
 (2) 
 
 where RR = relative risk, 
 
 AcC = accidents associated with category, 
 
 hC = hours worked in category, 
 
 TAc = total number of accidents, 
 
 and Th = total hours worked. 
 
 (The term "category," as used in this equation, refers to 
 location or the type of behavior associated with the acci- 
 dent and hours worked terms.) Conceptually, there is no 
 difference between the proportion of hours worked and 
 the number of hours worked. The field estimates of the 
 proportion of work time spent in various locations or 
 behaviors was substituted for the number of work hours; 
 the total work hours is then equal to unity. 
 
 The first comparison made was between direct behavior 
 and use of access system elements. Direct behavior has a 
 relative risk ratio of 1.5, while access systems have a rela- 
 tive risk ratio of 2.2. These ratios are presented in ta- 
 ble 7; the calculation of all relative risk ratios is included 
 in appendix A. Access systems are relatively more hazard- 
 ous than direct behavior. This effect is maintained when 
 the overlap cases are eliminated from the calculation of 
 the relative risk ratios. Based on the accident narratives, 
 there is an overlap of 18 cases between the 2 sets of ante- 
 cedent events: direct behavior and access system elements. 
 If these cases are eliminated, the relative risk ratios for be- 
 havior and access systems are 1.3 and 2.0, respectively. 
 
 Table 7.-Relative risk ratios for direct worker 
 behavior and access system elements 
 
 Category Risk ratio 
 
 Direct behavior 1.5 
 
 Access systems 2.2 
 
 Direct behavior 1 1.3 
 
 Access systems 1 2.0 
 
 Individual access system elements: 
 
 Ladders 6.8 
 
 Stairs and steps 2.5 
 
 Walkways .2 
 
 'Overlap cases eliminated. 
 
 The preliminary ratios in table 4 suggest that some ac- 
 cess elements, particularly ladders, are more hazardous 
 than others. Accordingly, relative risk ratios were cal- 
 culated for individual access system elements. These ratios 
 are also presented in table 7. Ladders have a very high 
 relative risk ratio of 6.8, stairs and steps have a high ratio 
 of 2.5, and walkways have a very low ratio of 0.2. The 
 relative risk ratios confirm the preliminary estimates of 
 the relative hazards of access system elements made in 
 table 4. 
 
 The greatest hazard antecedent to slip-and-fall accidents 
 is apparently the access systems, particularly ladders. 
 Reference to table 5 shows that, although improvement 
 has been noted, ladders are tied for the worst overall 
 ranking in terms of compliance with SAE J185 off-road 
 access system standards. Commonly noted ladder defici- 
 encies were the height of the first step above the ground 
 and unequal spacing of ladder rungs. 
 
 RISK PERCEPTION 
 
 The concept of subjective risk was introduced earlier in 
 this report. Subjective risk is defined as the individual's 
 experience of the hazard level associated with some behav- 
 ior. The field data gathered on worker behavior give some 
 understanding of the subjective experience of risk asso- 
 ciated with behavior. During the maintenance of the min- 
 ing equipment, six high-risk behaviors were observed dur- 
 ing a 105-min observation period. The individuals were 
 engaging in identified risk behaviors at a rate of one 
 behavior every 17.5 min. None of these behaviors resulted 
 in an accident. 
 
10 
 
 During the collection of field data, one slipping incident 
 was observed in approximately 80 h. This incident did not 
 result in an injury. 
 
 Earlier, the incidence rate of slip-and-fall accidents was 
 estimated as 0.58 per 100 work years for all surface mine 
 workers. Using the same method, the incidence rate of 
 slip-and-fall accidents for maintenance workers is esti- 
 mated as 1.98 per 100 work years. 
 
 An individual worker's subjective risk experience with 
 regard to hazardous behaviors would be this: A minor, 
 non-injurious slip might be expected, on average, after 
 enough time had elapsed for the occurrence of 274 haz- 
 ardous, or risk associated behaviors. An accident has a 
 probability of occurrence of 0.019 per work year. Thus, 
 using a binomial estimate, there is about an even chance 
 
 (p = 0.46) that a worker could work 40 years without ex- 
 periencing a slip-and-fall accident. (The calculations for 
 this probability are shown in appendix B.) It should now 
 be apparent that the subjective level of risk of engaging in 
 risk behavior is, justifiably, quite low. 
 
 Similar arguments can be made for estimation of the 
 subjective appreciation of the risk of using access systems. 
 The estimated incidence rate of slip-and-fall accidents 
 while using access systems is 0.46 per 100 work years. 
 (Calculations are shown in appendix B.) If this rate is 
 converted to an annual binomial probability of injury while 
 using an access system of 0.004, the probability of working 
 for 40 years without experiencing a slip-and-fall injury 
 while using an access system is 0.84. 
 
 SUMMARY AND CONCLUSIONS 
 
 The Bureau studied slip-and-fall accidents that occurred 
 during surface mining maintenance. A list of antecedent 
 events to slip-and-fall accidents was generated from analy- 
 sis of slip-and-fall accident narratives. 
 
 Two main variables were studied as they related to slip- 
 and-fall accidents during surface mining maintenance. 
 These variables are the direct behavior of the individuals 
 involved in the accident and the design of the access 
 systems. The use of access systems was found to have a 
 higher relative risk ratio than the direct, hazardous 
 behaviors identified from the analysis of the narratives. 
 Ladders were identified as the most hazardous of access 
 systems. Improvement in the compliance of new equip- 
 ment access systems with SAE J 185 guidelines was noted, 
 but the access systems of older equipment, particularly 
 off-road haul truck ladders, are still problematic. 
 Subjective risk values were estimated for maintenance 
 workers; the obtained values were quite low, suggesting 
 that individuals will routinely engage in risk behavior 
 without significant expectation of injury. 
 
 While the probability of a slip-and-fall injury is small 
 for any individual maintenance worker, it is cumulative for 
 a group. The probability of a maintenance worker sustain- 
 ing a slip-and-fall injury in a company with five mainten- 
 ance workers is 0.095 during any year. Thus, it is probably 
 more reasonable for a company to be more sensitive to 
 accident risk than an individual because the expectation of 
 loss is greater. 
 
 Given that a company is more likely to be sensitive to 
 the risk of slip-and-fall accidents, what strategies should be 
 taken to improve slip-and-fall safety? The first priority, 
 based on the findings of this study, would be to improve 
 the quality of access system elements, particularly ladders. 
 A reasonable approach might be to bring them into com- 
 pliance with the SAE J185 guidelines. The second priority 
 would be to make an effort to change the subjective risk 
 perception of the maintenance workers. Training pro- 
 grams have been described (20) that effectively teach 
 workers to recognize hazards and to change their behavior 
 accordingly. 
 
 REFERENCES 
 
 1. Long, D. A. An Analysis of Off-Highway Haulage Truck Main- 
 tenance and Repair Accidents, 1978-84. BuMines IC 9139, 1987, 15 pp. 
 
 2. Butani, S. J., and A. M. Bartholomew. Characterization of the 
 1986 Coal Mining Workforce. BuMines IC 9192, 1988, 76 pp. 
 
 3. . Characterization of the 1986 Metal and Nonmetal 
 
 Workforce. BuMines IC 9193, 1988, 69 pp. 
 
 4. U.S. Department of Labor. Mine Injuries and Worktime, 
 Quarterly. Closeout Ed., 1986, 31 pp. 
 
 5. Albin, T. J. Bureau Research in Human Factors Applied to 
 Recent Accident Experience in the North-Central Region. Pres. at 
 MSHA-IL Dep. Mines & Miner.-IL Assoc. Aggregate Prod. Saf. Conf., 
 Feb. 24, 1988, 6 pp.; available from T. J. Smith, Twin Cities Res. Cent., 
 BuMines, Minneapolis, MN. 
 
 6. Chi, D. N. H., and B. J. Hamilton. Trends in Mining Accidents 
 and Their Costs (1975-1984). Pres. at 19th Int. Symp. on Appl. of 
 Comput. Methods in Miner. Ind., University Park, PA, Apr. 14-18, 1986, 
 14 pp.; available from Librarian, Twin Cities Res. Cent., BuMines, 
 Minneapolis, MN. 
 
 7. U.S. Department of Labor. Mine Injuries and Worktime, Quar- 
 terly. Jan.-June 1987, 33 pp. 
 
 8. Kogut, J. An Empirical Bayes Methodology for Accident Risk 
 Estimation and Incidence Rate Comparison. MSHA, Health and Saf. 
 Anal. Cent., Denver, CO, 1982, 93 pp. 
 
 9. Lutz, R P. Discounted Cash Flow Techniques. Ch. in Hand- 
 book of Industrial Engineering, ed. by G. Salvendy. Wiley, 1984, 
 pp. 9.3.1-9.3.17. 
 
 10. Miller, J. M. The Management of Occupational Safety. Ch. in 
 Handbook of Industrial Engineering, ed. by G. Salvendy. Wiley, 1984, 
 pp. 6.14.1-6.14.18. 
 
 11. Quisenberry, S. Injuries Associated With Truck Ladders. MSHA 
 Health and Saf. Anal. Cent., Denver, CO, 1982, 36 pp. 
 
 12. Oppe, S. The Concept of Risk: A Decision Theoretic Approach. 
 Ergonomics, v. 31, 1988, pp. 435^40. 
 
 13. Howarth, C. I. The Relationship Between Objective Risk, Subjec- 
 tive Risk, and Behavior. Ergonomics, v. 31, 1988, pp. 527-535. 
 
11 
 
 14. Long, D. A. Improved Personnel Access for Surface Mining 
 Equipment. BuMines IC 8983, 1984, 20 pp. 
 
 15. Society of Automotive Engineers. Access Systems for Off-Road 
 Machines. SAE J185 (Aug85) in On-Highway Vehicles and Off- 
 Highway Machinery. SAE Handbook, v. 4, 1987, pp. 40.309^0.314. 
 
 16. Cohen, J. A Coefficient of Agreement for Nominal Scales. Educ. 
 and Psychol. Meas., v. 20, No. 1, 1960, pp. 37^6. 
 
 17. Ramsey, J. D., C. L. Burford, and M. Y. Berhir. Systematic 
 Classification of Unsafe Worker Behavior. Int. J. Ind. Ergonomics, v. 1, 
 1986, pp. 21-28. 
 
 18. Richardson, W. J., and E. S. Pape. Work Sampling. Ch in Hand- 
 book of Industrial Engineering, ed. by G. Salvendy. Wiley, 1984, 
 pp. 4.6.1^.6.21. 
 
 19. Silverstein, B. A. Patterns of Cumulative Trauma Disorders in 
 Industry. Eng. Summer Conf. in Occup. Ergonomics, Univ. MI, Ann 
 Arbor, MI, 1987, 10 pp. 
 
 20. Hopkins, B. L., R J. Conard, and M. J. Smith. Effective and 
 Reliable Behavioral Control Technology. Am. Ind. Hyg. Assoc. J., v. 47, 
 No. 12, 1986, pp. 785-791. 
 
12 
 
 APPENDIX A.-SAMPLE CALCULATIONS OF RELATIVE RISK RATIOS 
 
 Direct Behavior 
 
 (123/0.06) 
 
 2,050 
 1,338 
 
 = 1.5 
 
 = 2.2 
 
 (1,381 - 123)/(1 - 0.06) 
 
 Access Systems 
 
 (286/0.108) 2,648 
 
 (1,381 - 286)/(1.0 - 0.108) ™ 1,228 
 
 Direct Behavior Accidents With Overlap Removed 
 
 (105/0.06) 1,750 
 
 (1,381 - 105)/(1 - 0.06) : 1,357 
 
 Access System Accidents With Overlap Removed 
 
 (268/0.108) 2,481 
 
 (1,381 - 268)/(1.0 - 0.108) 1,248 
 
 = 2.0 
 
 Individual Access System Elements 
 Ladders: 
 
 (160/0.019) 
 
 (1,381 - 160)/(1.0 - 0.019) 
 
 Stairs and Steps: 
 
 (114/0.035) 
 
 (1,381 - 114)/(1.0 - 0.035) 
 
 Walkways: 
 
 (12/0.054) 
 
 8,421 
 1,245 
 
 3,257 
 1,313 
 
 222 
 
 = 6.8 
 
 = 2.5 
 
 (1,381 - 12)/(1.0 - 0.054) 1,447 
 
 = 0.2 
 
13 
 
 APPENDIX B.-CALCULATION OF PROBABILITY OF SLIP-AND-FALL ACCIDENT 
 HAPPENING TO INDIVIDUAL DURING 40-YEAR PERIOD 
 
 The probability that an individual will experience a slip- n years without a slip-and-fall accident is 0.981 raised to 
 
 and-fall accident during any year is 0.019. Thus, the prob- the nth power. The probability of working 40 years with- 
 
 ability that a slip-and-fall accident will not happen is out a slip-and-fall accident is thus 0.981 raised to the 
 
 1 - 0.019 = 0.981. Each year is assumed to be indepen- 40th power, or 0.46. 
 dent of every other year. The probability of working 
 
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