The Epidemiology of Slips, Trips, and Falls in a Helicopter Manufacturing Plant

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1 SPECIAL SECTION: Occupational Fall Prevention and Protection The Epidemiology of Slips, Trips, and Falls in a Helicopter Manufacturing Plant Harlan Amandus, Jennifer Bell, Hope Tiesman, and Elyce Biddle, National Institute for Occupational Safety and Health, Morgantown, West Virginia Objective: The purpose of this evaluation was to evaluate the causes and costs of slips, trips, and falls (STFs) in a helicopter manufacturing plant. Background: STFs are a significant portion of the total industry injury burden. Method: For this study, 4,070 helicopter plant workers who were employed from January 1, 2004, through February 28, 2008, were enrolled. Company records on workers compensation claims, occupational health first report of injury, and payroll records on hours worked were collected. Cause and source of all injuries, including STFs, were coded for analysis. Results: During the 4-year study period, there were 2,378 injuries and 226 STFs (46 falls [20%] to a lower level, 117 [52%] falls on the same level, 41 [18%] from loss of balance without a fall, and 22 [10%] from other events). Of the 226 STFs, 123 falls to the same level were caused by slippery substances (52), objects on floor (43), and surface hazards (28), and they cost $1,543,946. Falls to lower levels primarily involved access to stands to and from aircraft and falling off large machines. Conclusion: More than half of the STF injury claims likely could have been prevented by housekeeping and maintenance, and this cost saving could reasonably offset a considerable portion of the cost of preven tion. Training and stand modifications could be considered to prevent falls from elevation from stands, machines, and aircraft. Application: Recommendations for STF prevention are discussed. Keywords: causes and costs, helicopter plant, slips, trips, falls Address correspondence to Harlan Amandus, Division of Safety Research, National Institute for Occupational Safety and Health, 1095 Willowdale Road, M/S 1811, Morgantown, West Virginia 26505; hamandus@cdc.gov. Author(s) Note: The author(s) of this article are U.S. government employees and created the article within the scope of their employment. As a work of the U.S. federal government, the content of the article is in the public domain. HUMAN FACTORS Vol. 54, No. 3, June 2012, pp DOI: / INTRODUCTION Work-related slips, trips, and falls (STFs) contribute a significant portion to the national burden of injury and workers compensation (WC) costs (Cotnam, Chang, & Courtney, 2000). In 2008, STF events, which include falls to a lower level, falls on the same level, and slips, trips, and loss of balance without a fall, were the second most prevalent (24.2%) cause of lostworkday (LWD) injury among U.S. workers in private industry (excluding government workers), following injuries caused by contact with objects (27.1%) (Bureau of Labor Statistics, 2009a). In 2008, there were 260,610 privateindustry work-related STF injuries, 72,250 in transportation and utilities, 52,030 in education and health services, 28,530 in construction, and 28,430 in manufacturing. In the aircraft manufacturing industry, STFs are the third leading cause of LWD injury (accounting for 17.6% of the total), following musculoskeletal injuries attributable to overexertion and repetitive motion (45.3%) and injuries related to contact with objects (18.5%) (Bureau of Labor Statistics, 2009b). In an effort to address work-related injury in the aircraft manufacturing sector, a collaborative project was initiated between the National Institute for Occupational Safety and Health (NIOSH) and a helicopter manufacturing company to evaluate the company safety program s success in reducing injuries in one of its primary plants. The NIOSH project team worked with the company to benchmark baseline injury numbers and rates prior to implementation of safety interventions according to primary work activity. The purposes of this baseline analysis are (a) to determine the leading causes of STFs in the plant; (b) to determine the indemnity and medical costs associated with the different causes of STF injuries; (c) to estimate rates of STFs associated with employee factors, such

2 388 June Human Factors as activity group, age, gender, and length of employment; and (d) to make recommendations for STF prevention in the plant. METHOD The study population consisted of 4,070 production workers who were employed from January 1, 2004, through February 28, 2008, at a helicopter manufacturing plant. The plant employs approximately 3,500 production workers and 3,000 engineers and administrative office workers. Engineering, administrative, and executive management workers were excluded from the study, as safety interventions focused on production workers. Plant departments include blades and composite manufacturing, milling and machining, bench assembly of parts, avionics assembly of wire harnesses, gears and transmission machining, sheet metal fabrication, helicopter final assembly, painting and finishing, hangar and flight mechanics, upholstery, security and fire department, and warehouse and materials moving. For this analysis, activity groups were formed from departments and activities crossing departments. Data for this analysis were supplied by the helicopter manufacturing plant and included (a) payroll records on dates and hours worked for all employees; (b) personnel records on date of birth, gender, occupation, department, and dates of hire and termination; (c) WC claim records, including dates, cause, nature of injury, body part, and costs of all compensable injuries; and (d) company physician and nurse records on first report of injury pertaining to diagnosis or nature and cause of injury. Data on first report of injury were available on all injuries, including those leading to WC claims. Each WC claim record was merged with its corresponding record of first report of injury for coding event and source of injury. Using the narrative text fields from the combined WC claims and records of first report of injury, we coded the event and source of all injuries using the Bureau of Labor Statistic s (1992) Occupational Illness and Injury Classification system (OIICS) by a trained OIICS coder. STFs in this analysis included all falls (OIICS single-digit Code 1) and slips, trips, and loss of balance without a fall (OIICS Code 215), such as when a person slipped, tripped, misstepped, stumbled, or fell against an object but did not fall. To supplement the OIICS event code, the cause of the first initiating event of a STF was additionally coded as a slip, trip, fall, loss of balance, or unknown (Bell et al., 2008; Bentley & Haslam, 1998). Additional codes to supplement the OIICS source code were developed (see Table 1 for taxonomy) to provide specificity for STF exposures in the plant similar to the approach used by Bell et al. (2008). The company is self-insured and, thus, pays for its WC costs. Data on all WC medical and indemnity costs from all claims filed during the study period on the study population were used in the cost analysis. For this analysis, the total cost of WC claims, average cost per injury, and average cost per claim were calculated for the study period January 1, 2004, through February 28, Costs were not discounted or adjusted by year. Injury rates per 10,000 full-time equivalents (FTEs) were computed for comparison with those for the general manufacturing and aircraft industry (Bureau of Labor Statistics, 2009a). Rates were also computed for all STFs and LWD STF injuries. LWD injury is an injury causing a worker to miss 1 or more days of work. Computation of rates assumed an FTE was 2,000 hr of work per year. The rate per 10,000 FTEs was computed as the number of STFs divided by the number of person-hours worked times 20,000,000 hr. The association between STF rates and age, gender, and tenure was tested with the use of a Z test approximation to a binomial exact test for incidence density data (Kleinbaum, Kupper, & Morgenstern, 1982). For this analysis, the number of STFs and person-hours worked were calculated for each year from 2004 to 2008 by age (younger than 45, 45 or older), computed at midyear (June 1); tenure (less than 1 year, 1 year or more), computed at the end of the year; and gender (male, female); and then were summed across all of the years. RESULTS STFs by OIICS Event and Source There were 226 STFs among the 4,070 helicopter plant production employees who had

3 TABLE 1: Frequency of Slips, Trips, and Falls by Source and Activity Group Source Activity Group Aircraft Stand or Stairs On Level Ground Machine Chair or Stool Ice or Snow Other Slippery Substance Slippery Object Object on Floor Furniture Surface Hazard Other Total Hangar Final assembly Bench assembly Machining Blades Maintenance QA large parts Avionics Paint Other Total Note. Aircraft = climbing off, onto, into, or out of aircraft stand; stand or stairs = working on level on stand or walking up or down stairs of building or stands; on level ground = walking or standing on level ground; machine = standing on machine or equipment; chair or stool = sitting or standing on chair or stool; other = all other sources; QA = quality assurance. 389

4 390 June Human Factors TABLE 2: Frequency and Percentage of All Slips, Trips, and Falls (STFs) and Lost Workday (LWD) STF Injuries by Source Source All STFs LWD STFs n % n % Slippery substances on floor Ice and snow Oil Water Food Painted surface Other slippery substance Slippery objects (gravel, wet pad on floor, creeper) Other objects on floor Parts (screws, tools, blocks, boxes, equipment, skids) Hose (hoses, tubing, air and hydraulic lines, pipe) Cable (cable, wire, cords) Other objects on floor Furniture (trips over cabinet, chair, stools, equipment) Surface hazard Mat or rug edge Hole or crack in surface (sidewalks, grates, stand, walkway) Elevator (uneven elevator and building floor) Other surface irregularity Chair or stool (sitting or standing on chair or stool) On level ground when walking or standing Stepping or climbing off, onto, into, or out of aircraft stand On machine or equipment (standing on) Stairs of building or stands (walking up or down) On level stand (working or standing on stand) Other Total worked from January 1, 2004, through February 28, Of the 226 STFs, 23 (10.2%) employees had at least 1 LWD injury. Among the 2,378 total injuries that occurred during this period, STFs were the fourth leading OIICS event (9.5%), following bodily reaction and exertion excluding injuries caused by slip, trip, or loss of balance without a fall (38.4%); contact with objects (34.6%); and exposure to harmful substances (11.3%). Among the 133 LWD injuries that occurred during this period, STFs were the second leading OIICS event (17.3%), following bodily reaction and exertion (54.9%). Of the 226 STFs, the most frequent OIICS events were falls on the same level (117 injuries, 51.8%), followed by falls to a lower level (46 injuries, 20.4%), and slips, trips, or loss of balance without a fall (41 injuries, 18.1%). The most frequent OIICS events among falls to a lower level were falls from a nonmoving vehicle (9 injuries), followed by falls down stairs or steps (6), falls from scaffolds or staging (4), falls from ladders (2), and falls from the floor, dock, or ground (1). The most frequent events among falls on the same level were falls to the floor, walkway, or other surface (77 injuries), followed by falls onto or against objects (31). Of the 226 STFs, 55 were attributable to a slip, 72 to a trip, 22 to a fall, and 70 to loss of balance. The leading source of STFs (Table 2) was slippery surfaces, which contributed to 52 (23.0%) STFs, followed by objects on the floor, which contributed to 43 (19.0%); walking on a level ground (32, 14.2%); surface hazards (28, 12.4%); climbing into, out of, onto, or off of the aircraft from or to a stand (17, 7.5%);

5 Epidemiology of Slips, Trips, and Falls 391 TABLE 3: Frequency of Slips, Trips, and Falls (STFs); Number of Person-Hours Worked (PH), and Rate per 10,000 Full-Time Equivalents (FTEs) of STF by Activity Group Activity Group STFs PH Rate [CI] Rate Ratio Z Value p Value Hangar 21 1,448, [165.9, 414.0] Final assembly 59 5,153, [170.6, 287.4] Bench assembly 9 815, [76.5, 364.7] Machining 32 3,049, [137.2, 282.6] Blades 26 2,480, [129.1, 290.3] Maintenance 4 402, [4.0, 394.0] QA large parts 6 634, [37.8, 340.3] Avionics 12 1,361, [76.5, 276.0] Painting 5 615, [20.1, 304.9] Other 52 7,777, [97.3, 170.1] 1.0 Total ,738, [165.6, 215.2] Note. See text for definition of rate and FTE. CI = 95% confidence interval for rate; QA = quality assurance. Z value and p value from Z test on exact test comparing rates in an activity group with rates in the other group. standing on machines or equipment (8, 3.5%); and walking up or down stairs (8, 3.5%). The leading source of slippery substances was ice or snow, which contributed to 30 of the 52 STFs on slippery surfaces, followed by oil, which contributed to 14. Of the total, 11 cases were attributable to falling while seated or standing on a chair or stool. Sources of LWD STFs were similar to that of all STFs. STFs by Event, Source, and Activity Group The highest rate of STFs was among workers in the hangar area, followed by final assembly, machining, and blades assembly areas (Table 3). Rates were significantly higher (p <.05) in these areas than in the other group of activities. The other group consisted of activity groups with fewer than 1 STF during the study period. The largest number of falls to a lower level occurred in the final assembly area (15), followed by hangar (6), machining (5), and blades assembly areas (5). In the final assembly area, there were 14 falls to a lower level involving stands that were moving during exit of the aircraft (falling off the aircraft and falling off stands; see Table 1). In the machining area, there were 4 falls involving standing on a machine or equipment. Falls from chairs or stools occurred most frequently in areas where workers used chairs or stools for bench work or aircraft assembly, such as in the hangar, final assembly, bench assembly, and blades assembly areas (Table 1). Falls to the same level occurred in most areas and contributed to 50% to 100% of the STFs among the activity groups. Slippery substances, objects on the floors, and surface hazards contributed to 123 STFs and contributed to 44% to 100% of all STFs among activity groups (Table 1). STF Rates by Age, Gender, and Tenure There was no significant difference (i.e., p >.05) between STF rates per 10,000 FTEs between workers 45 years or older compared with those younger than 45 (rate ratio = 0.96; see Table 4). Rates for 10-year age groups were for ages <20, for ages 20 to 29, for ages 30 to 39, for ages 40 to 49, for ages 50 to 59, and for ages 60 and older. Rates were lowest for ages younger than 20 and between 30 and 39 and highest for ages 60 and older. However, the difference in rates was not significant between those younger than 20 and those 60 and older (rate ratio = 1.22) or between those younger than 20 and other 10-year age groups (rate ratios range from 1.09 to 1.10). Females had significantly higher STF rates than males (rate ratio = 1.55; Table 4). However, there was no association between source (slippery substances, objects on floor, surface hazards, and other sources) and gender (p =.95 from chi-square contingency test). Finally, there

6 392 June Human Factors TABLE 4: Number and Rate per 10,000 Full-Time Equivalents (FTEs) of Slips, Trips, and Falls by Age, Gender, and Tenure Variable Frequency Person-Hours Rate [CI] Rate Ratio Z Value p Value Age < ,086, [141.3, 231.2] ,652, [162.4, 221.9] 0.96 Gender Male ,304, [150.5, 202.1] Female 47 3,434, [195.5, 352.0] 1.55 Tenure <1 year 7 972, [37.3, 250.5] years ,766, [166.9, 217.9] 1.34 Note. See text for definition of rate and FTE. Tenure refers to years worked in plant. CI = 95% confidence interval for rate. Rate ratio: 1.00 indicates reference group. Z value and p value from Z test of rate comparisons. TABLE 5: Total Cost, Average Cost per Injury, and Average Cost per Claim by Source of First Initiating Event of Slips, Trips, and Falls (STFs) Source Total Cost No. of STFs Cost per STF No. of Claims Cost per Claim Range of Cost Median Cost Slippery surface $856, $16, $35,668 $66 - $463,223 $4,116 Objects on $522, $12, $24,877 $400 - $91,435 $10,047 floor Surface hazards $165, $5, $16,549 $617 - $48,805 $10,215 All others $694, $6, $13,898 $104 - $105,147 $2,845 Total $2,238, $9, $21,322 $66 - $463,223 $4,546 Note. Claim = workers compensation claim with a cost of $1 or more. was no significant difference in rates between employees with tenure of 1 year or more and those with less than 1 year (rate ratio = 1.34). Cost per Injury The total WC and indemnity costs of STFs from January 1, 2004, to February 28, 2008, was $2,238,850, which was 16% of the cost of all injuries ($13,836,424). STFs had the second highest cost of all primary OIICS event injury categories, following the bodily reaction and exertion excluding injuries caused by slips, trips, or loss of balance without a fall ($8,575,108). However, falls had the highest average cost per injury ($10,108) and cost per claim ($22,532). Among categories of fall events, falls on the same level had the highest total cost ($1,405,302), average cost per injury ($12,011), and average cost per claim ($26,515). The total cost was highest for STFs on slippery surfaces ($856,043), followed by all others ($694,905), objects on the floor ($522,408), and surface hazards ($165,495) (Table 5). Among events caused by slippery surfaces, slipping on ice accounted for 30 (58%) of the 52 STFs in this category and cost $698,012 (82% of the cost of all STFs caused by slippery surfaces), an average cost per injury of $23,267 and cost per claim of $53,693. DISCUSSION Worker slip, trip, and fall injury patterns in the aircraft manufacturing industry have not been previously described in the literature in detail. This article provides additional support for the importance of good housekeeping and routine maintenance in the prevention of industrial slip, trip, and fall incidents. More than half

7 Epidemiology of Slips, Trips, and Falls 393 of all STF injuries in this aircraft manufacturing facility were caused by slippery surfaces, objects left on the floor, and surface hazards, all of which could potentially be mitigated by improved housekeeping and maintenance. This is not a surprising result and has been found in other industries (Bell et al., 2008; Cohen & Compton, 1982; Kemmlert & Lundholm, 2001; Manning, Ayers, Jones, Bruce, & Cohen, 1988). It is not well known from the published literature that the costs of STFs are so high and, thus, that savings through their reduction would go a long way in their prevention. Even a modest reduction in STFs from increased housekeeping and grounds maintenance services could result in a considerable reduction in STF injuries and savings to the company. The cost of the 123 STF events ($1,543,946) attributable to slips on slippery substances and trips on objects and surface hazards could offset a large part of the cost of housekeeping and maintenance services. For example, the total cost of STFs on ice and snow ($698,012) would likely offset part of the additional cost of ice-melting treatments for sidewalks and walkways more than that already being spent by the company, $165,495 for STFs from surface hazards may offset the cost of some materials and labor for repairs to floors and walking surfaces, and $1,378,451 for STFs from slippery surfaces and trips over objects would likely offset part of the cost of the staff time for site inspections, housekeeping, and training and promotion of awareness to keep substances and objects off the floor. Bell et al. (2008) demonstrated a 58% reduction in STF rates in three hospitals from a prevention program most heavily weighted toward housekeeping and maintenance. Thus, it is logical to assume that the helicopter plant could possibly realize a similar reduction in STFs from an effective housekeeping and maintenance program and, thus, a cost savings of approximately $1.3 million (58% $2,238,850 cost of STFs) after 4 years. A business case analysis would be needed to determine more accurately the return on investment in an increased housekeeping and maintenance program. Among the 226 STFs, 32 STFs (14.2%) involved loss of balance or missteps while walking or standing on the level ground and 15 (3.5%) involved falls or loss of balance while walking up and down stairs in the building or on an aircraft stand or while working on a level stand. The cause of these STFs is unknown. More investigation into loss of balance and missteps could be done to investigate risk factors, such as risky activities and surface characteristics. Additionally, 17 (7.5%) STFs were caused by a fall or loss of balance or misstep while climbing off, onto, into, or out of aircraft, and 8 (3.5%) occurred while standing on machines or equipment. The current training procedures and materials on these activities should be reviewed, and improvements in the training on access and movement in and on aircraft, stands, machines, and stairs and stands should be considered. Several injuries occurred, for example, when a stand moved while someone was climbing off of or out of the aircraft. Training or marking stands being used by workers for access from the aircraft and modification of stands to prevent stand movement could also be considered. An evaluation of machine access points could also be considered. Finally, 11 STFs occurred when chairs slipped out from under an employee while sitting or when standing on a chair or stool. Employees doing bench work in the avionics (wiring harness assembly area) and other bench assembly areas use ergonomic chairs that can be raised and lowered to access the workbench. Evaluation of the center of gravity for these chairs should be investigated and, possibly, workers alerted to the problem of chairs slipping out from under them. Training could also be considered to encourage employees to not use a chair as a ladder in addition to providing easy accessibility to stepstools and ladders. Age, gender, and tenure have been shown in other studies to be associated with STF rates, but an association with age and tenure was not found in this study. Some studies have shown a higher risk of STFs among workers older than 40 to 45 years of age compared with younger workers (Bell et al., 2008; Buck & Coleman, 1985; Davies, Stevens, & Manning, 2001; Kemmlert & Lundholm, 2001), and others have shown higher rates in younger workers (Layne & Pollack, 2004; Leamon & Murphy, 1995). Results of this study indicated that there was no

8 394 June Human Factors significant difference in STF rates between age groups, although rates were slightly lower among workers younger than 20 and between 30 and 39 and slightly higher among those 60 and older. Underreporting of minor injuries, particularly among younger workers, may possibly explain differences among studies. However, underreporting unlikely explains difference among studies with respect to more severe injuries, such as LWD injuries. Some studies have shown a higher STF rate among females than males (Bell et al., 2008; Davies et al., 2001), and others have shown a higher rate among males (Layne & Pollack, 2004; Leamon & Murphy, 1995) or a higher proportion of all occupational injuries caused by STFs among males (Kemmlert & Lundholm, 2001). Results of this study showed that females had an STF rate 1.6 times higher than males. Although females had a slightly higher percentage of STFs involving slippery substances, objects on the floor, and irregular surfaces, the association between gender and source of STFs was not significant. Studies have also shown that more recently hired workers have STF rates twice as high as or higher than those who have longer tenure (Bell et al., 2008; Breslin & Smith, 2006). Results of this study showed no significant difference in STF rates among those with less than 1 year and 1 or more years of employment (rate ratio = 1.3). However, the helicopter plant workforce had a low attrition rate, and only a very small number of workers had been employed less than 1 year. LWD STF rates in the helicopter plant were similar to the average rates in the U.S. manufacturing and aircraft industries. The average annual rate for 2004 to 2007 in the study plant per 10,000 FTEs for LWD injuries was 5.8 for falls to a lower level, 8.4 for falls to the same level, and 5.0 for slips, trips, or loss of balance without a fall. Comparable LWD average annual rates for 2004 to 2007 in the manufacturing industry were 5.7, 13.3, and 4.8, respectively (Bureau of Labor Statistics, 2009a). Comparable average annual rates for 2006 to 2007 in the aircraft manufacturing industry were 3.7, 7.1, and 5.7, respectively (Bureau of Labor Statistics, 2009b). In comparison with other industries, the aircraft manufacturing industry has a relatively low overall rate of LWD injuries (82.6 per 10,000 FTEs, compared with per 10,000 FTEs for the all-industry average), as reported by the U.S. Bureau of Labor Statistics (2009b). However, prior research within this industry (Neitzel, Seixas, Harris, & Camp, 2008) has shown that there can be significant variation between manufacturing plants as to the degree of noncompliance with fall protection measures. Given the potential severity of these events and the large WC injury burden described in this study, recommendations are as follows for prevention of STFs in the helicopter plant: 1. Conduct an audit of the plant of slippery substances, surfaces in need of repair, and objects on the floor. 2. Repair surface hazards, including holes or cracks in grates, stands, or other walking surfaces; uneven surfaces; mats and rugs with elevated edges; uneven elevator floor with building floor; and so on. 3. Conduct an audit and repair drainage from gutters that may create ice on walkways. 4. Increase monitoring of walkways for ice and snow, and increase ice-melting chemical applications in walking areas. 5. Increase awareness among workers to clean up spills from slippery substances immediately and to remove parts and tools from floors as soon as possible. Supervisors and safety officials should conduct periodic audits to monitor the workplace for these hazards. 6. Implement a plant promotional campaign to increase awareness of slippery substances and objects on the floor. 7. Conduct a business case analysis on the return on investment associated with the cost of these interventions assuming significant reductions in preventable injuries. 8. Evaluate steps and access points on machines to prevent falls. 9. Consider training in access to and around the stand to and from aircraft. 10. Consider use of signs to hang on stands so others will not move them and an evaluation and modification of stand mounts to prevent movement during access. 11. Evaluate center of gravity of ergonomic chairs among the STFs associated with falling off chairs. Provide training to employees in bench areas to

9 Epidemiology of Slips, Trips, and Falls 395 be cautious of leaning and tipping chairs while sitting and the risk of the chairs slipping out from under them. Training should emphasize not using chairs or stools as a stepladder. In conclusion, most, if not all, STFs are preventable. The cost of 54% of STF injuries that are preventable by housekeeping and maintenance could offset a large part of the cost of an increased housekeeping and maintenance program. Recommendations are likely applicable to other aircraft and general manufacturing operations. ACKNOWLEDGMENTS The authors would like to acknowledge the many hours of computer programming provided by David Hilling and Nan Wu to complete this article. KEY POINTS Of the 226 slips, trips, and falls that occurred from January 1, 2004, through February 28, 2008, in a helicopter manufacturing plant that were attributable to slippery surfaces, objects on the floor, and surface hazards, 54.4% were preventable by housekeeping and maintenance and cost $1,543,946. Considerable savings could likely be realized through prevention efforts. REFERENCES Bell, J. L., Collins, J. W., Wolf, L., Gronqvist, R., Chiou, S., Chang, W., Sorock, G. S., Courtney, T. K., Lombardi, D. A., & Evanoff, B. (2008). Evaluation of a comprehensive slip, trip and fall prevention programme for hospital employees. Ergonomics, 51, Bentley, T. A., & Haslam, R. A. (1998). Slip, trip and fall accidents occurring during the delivery of mail. Ergonomics, 41, Breslin, F. C., & Smith, P. (2006). Trial by fire: A multivariate examination of the relation between job tenure and work injuries. Occupational and Environmental Medicine, 63, Buck, P. C., & Coleman, V. P. (1985). 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He received his PhD in epidemiology from the University of North Carolina, Chapel Hill, in Jennifer Bell is a research epidemiologist in the Division of Safety Research at the National Institute for Occupational Safety and Health. She received her PhD in forest resources science from West Virginia University in Hope Tiesman is a research epidemiologist in the Division of Safety Research at the National Institute for Occupational Safety and Health. She received her PhD in epidemiology from the University of Iowa in Elyce Biddle is a senior economist in the Division of Safety Research at the National Institute for Occupational Safety and Health. She received her PhD in occupational safety and health from West Virginia University in Date received: October 22, 2010 Date accepted: February 7, 2011