BJS: Bureau of Justice Statistics

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Bureau of Justice Statistics (BJS)
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Home  | FAQ Detail
FAQ Detail
Why are data so difficult to compare over years?

Caution must be used when using trend data, as definitions and reporting capabilities change over time. Some changes in definitions are due to BJS initiatives to improve counting (such as separating out state inmates held in private facilities or local jails), some may be driven by the Office of Management and Budget (such as changes in racial and ethnic definitions), and some may be noted by the reported jurisdiction (such as noncitizen inmate counts, including those who were foreign-born). 

Whenever possible, BJS notes these differences and encourages users to check footnotes within tables and jurisdiction notes within reports to better understand why comparability can vary from state-to-state or year-to-year.

Note:  When you see a sharp increase or decline in a year-to-year count, it is recommended to verify there was no change in definition or counting method.

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