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3 STATISTICAL ANALYSES OF SMALL UNDERGROUND COAL MINES Introduction In its report Toward Safer Underground Coal Mines, the Committee emphasized that smaller mines, in particular mines with 50 or fewer employees, were found to have a fatality rate nearly three times higher than that in mines with over 250 employees, and nearly twice that of mines with 51-250 employees. This strong correlation between mine size and fatality rates was evident in all the data from the Mine Safety and Health Administration we examined dating back to 1969. Furthermore, the association was not explainable by company ownership, union status, seam thickness, or any of the other factors we examined. It was also seen that the distribution of types of accidents causing fatalities did not vary much across mine size. This indicates that the larger fatality rate in small mines is not the result of an increase in a specific type of accident (e.g., roof falls). Rather, the data indicated that smaller mines are more likely than larger mines to have fatalities from each of the major types of accidents. This would suggest that the problem in small mines is not isolated to a specific work activity (such as roof bolting), but is present in all aspects of the mining effort. One explanation (considered in our earlier report) of the association between mine size and fatality rates is that in large mines there are proportionally more workers away from the working face and therefore at reduced risk for a fatality. If this were true, then smaller mines would have larger fatality rates even though the risks for miners at the working face were the same as in larger mines. We allowed for this possibility in our analyses (see pp. 91-93) but found that, at best, it could explain only a part of the strong association between mine size and fatality rates. The present report summarizes the Committee's additional analyses of the relationship between mine size and fatality rates. As in our earlier report, we focus primarily on information collected from MSHA accident/injury and employment/production reports. We begin by examining features of small mines that distinguish them from larger mines and looking at how this might bear on their chances for fatal injuries. We then carry out analyses to determine whether and how much these factors help to explain the association.
Characteristics of Small Mines Fatality Rates by Size Table 1 gives a breakdown of fatality rates (R^)* by mine size for the period 1975-80. The category 1-50 employees, used in the Committee's earlier report, has been divided into three subcategories: 1-10, 11-20, and 21-50 employees. A finer gradation of the 1-10 category is not warranted, because of both the small numbers of employee-hours of risk and an apparent bias in the reporting practices of very small mines (to be discussed later). TABLE 1 Fatality Rates by Mine Size, 1975-80 (standard errors are in parentheses) Mine Size Million Fatality (number of No. of Employee- Rate employees) Mines Fatalities Hours (Rl) 1-10 1,683 26 24.4 .21 ( .04) 11-20 1,150 53 55.2 .19 ( .03) 21-50 700 58 111.0 .10 ( .01) 51-150 324 139 232.9 .12 ( .01) 151-250 133 85 223.0 .08 ( .01) 250+ 149 151 610.8 .05 ( .01) Note that mines with 20 or fewer employees have a fatality rate (.20) about twice that of mines with 21-50 employees. Thus the trend of increasing fatality rate with decreasing size continues within the 1-50 category. Table 2 gives a breakdown of mines by state and size. The table includes only those seven states with at least 20 million hours of underground coal mine employment during 1978-80. For each state the table gives the number of mines in each size category and the proportion of the state's employee-hours accounted for by that category. For example, in Kentucky there are 434 mines having from 1 to 10 employees, and these mines account for 4 percent of Kentucky's employee-hours. The last column of the table gives the percentage of each state's total hours accounted for by mines with 1-50 employees. *Defined as the number of fatalities per 200,000 employee-hours or during one year for 100 full-time workers. The estimated standard errors, given in parentheses, are computed as R]_//n, where n is the number of fatalities.
