Abstract
There are over 4,000 career or mostly career fire departments in the United States. Representing only 14 % of U.S. fire departments, these departments nonetheless protect 61 % of the population.
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“The sample of departments was determined in two steps—(a) identification of ‘selfrepresenting’ departments, and (b) the selection of the remaining ‘non-selfrepresenting’ departments. Self-representing departments are those whose populations served (and thus annual fire/EMS event volume) is so large that they would appear in the sample with certainty. To understand why self-representing departments are unavoidable in our probability-proportional-to-size (pps) sample, note that a sample of 494 departments would cover in aggregate the 227 million population served. Accordingly, each sampled department will ‘represent’ (227 million)/494 or about 460,000 population. Thus any department whose population served exceeds 460,000 would be sampled with certainty. It is conventional in survey sampling to use 75 % of this ratio as a threshold for determining self-representing selections. We used this approach to identify 76 self-representing departments, all with ‘population served’ exceeding 350,000.
“The remaining 418 non-self-representing departments were stratified by population served and sampled with probabilities proportional to population served. Combined with the 79 self-representing departments, this yielded our total desired sample size of 494.” (Averill, et al. 2008, 57)
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© 2010 Fire Protection Research Foundation
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Upson, R., Notarianni, K.A. (2010). Recruiting Participants. In: Quantitative Evaluation of Fire and EMS Mobilization Times. SpringerBriefs in Fire. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-4442-8_3
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DOI: https://doi.org/10.1007/978-1-4614-4442-8_3
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