Definition of the Target Population
The target population for a survey is the entire set of population units about which the survey data are to be used to make inferences. Establishment surveys such as the National Survey of the Mining Population must delineate the level of the business organization that constitutes the units of the target population. Because hazards vary across mines, the target population for this survey was defined in terms of the individual mining operation.
The target population of mines consisted of active mines in current production. The survey was further restricted to operations that were covered under Title 30 of the U.S. Code, specifically mines whose mineral output was sold or used in commerce. The target population of employees was restricted to those mine employees for whom the mine operator must report hours worked using the MSHA Form 7000-2: Quarterly Mine Employment and Coal Production
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Report (Appendix C). This includes all direct employees working at the mine, but not contract employees brought in periodically or regularly to perform work at the mine.
There is an important temporal aspect to these definitions for mines and for mine operator employees. Over time, some mines will go in and out of operation. Similarly, employees join the mining labor force and leave the labor force over time. Accordingly, the National Survey of the Mining Population focused on mines in operation during a particular calendar quarter and the current employees of those mines.
Construction of the Sampling Frame
The sampling frame for a survey is the list or mechanism used to enumerate target population members for sample selection purposes. Individual sampling frames for each of the five major mining sectors (see Figures 1–5) were constructed using the 2007 second quarter data released by the Mine Safety and Health Administration, so that the sampling frames would be in sync with the actual time period when data collection would begin (the second quarter of 2008).
To ensure that any startup or intermittent mining operations would not be missed, all mines reporting zero employment hours were included in these frames. Any mines with a status of abandoned or abandoned/sealed were excluded from the sampling frames. The Standard Industrial Classification (SIC) for the active coal, metal, nonmetal, stone, and sand and gravel mines used in the sampling frames is presented in Appendix D.
Stratification Guidelines
For the National Survey of the Mining Population, mine-level and employee-level analyses were planned, which required adequate sample sizes of mines and of employees.
Because multiple employees were to be sampled from each responding mine, sample size
requirements for mine-level analyses tended to drive the total number of mines that needed to be sampled. The sample size for employees was determined by the number of sampled mines responding and the average number of employees sampled per mine.
The competing needs of mine-level analysis versus employee-level analysis required the use of a compromise design that supported the objectives of both types of analyses. For mine-level analyses, the best design was one that selected mines with equal probability, while for employee-level analyses the best design was one that selected mines with probability
proportional to the number of employees. The compromise design met both needs by stratifying by the number of employees and then sampling mines with equal probability within strata. Strata associated with large mines (in terms of employment) were given greater selection probabilities than small mines, which would facilitate employee-level analyses by making the employee selection probabilities less variable across mines.
Mine size was an important domain for study at the mine level as well as at the employee level. For example, mines might be more likely to vary in their training procedures based upon employee size. Small mines may be more likely to use trainers from outside the organization, while large mines may be more likely to rely on in-house trainers. Hence, stratifying by the number of employees when sampling mines served an analytic purpose, as well as facilitated the over sampling of large mines needed for employee-level analyses (see Figure 6).
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From an analysis standpoint, it was also desirable to control for underground versus surface mines when sampling mines and employees (see Figures 7 and 8). Underground coal mines, in particular, have higher injury and fatality rates than surface mines. There is substantial diversity in the incidence of injuries and fatalities at underground mines versus surface mines across all mining sectors. Nearly one-third of coal and metal mines are underground. Less than ten percent of nonmetal and stone mines are underground and sand and gravel mines are surface only. Stratification by underground mines versus surface mines allows for the control over sample sizes needed for effective comparisons of underground mines to surface mines. A more in-depth discussion of the stratum size formation and sample size guidelines used in this survey can be found in Appendix E.
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Note: Mines spotted randomly within counties Not shown: Puerto Rico and U.S. Virgin Islands
Data Source: MSHA
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