"Every 3 to 4 years, the Bureau of Justice Statistics (BJS) surveys a nationally representative sample of state and local law enforcement agencies. The surveys are conducted as part of the Law Enforcement Management and Administrative Statistics (LEMAS) program. This report presents data from the 2007 LEMAS survey describing local police departments in terms of their personnel, budgets, operations, policies and procedures, computers and information systems, and equipment. Comparisons are made with prior years where appropriate, and as data are available. The selected local police sample includes all departments employing 100 or more full-time sworn personnel and a systematic random sample of smaller agencies stratified by size. (See Methodology for more information.) The sample was selected from a BJS census of law enforcement agencies […] Because sampling was used for smaller agencies, some of the statistics presented in the report are subject to sampling error. Statements of comparison have been tested at the 95%-confidence level. In order to compare departments with others serving similar sized jurisdictions, the statistics in this report are presented mainly by categories of population served. Because most departments employ small numbers of officers and serve small populations, overall agency-based percentages tend to reflect smaller departments more than larger ones. To adjust for this effect, additional overall percentages weighted by the number of officers employed are presented in the text and selected figures."
Bureau of Justice Statistics: http://bjs.ojp.usdoj.gov/