The Bureau of Justice Statistics (BJS) periodically conducts census data collections from all local jails in the United States. In years between census collections, BJS conducts the Annual Survey of Jails (ASJ). The survey collects data annually from a sample of local jails and uses the data to estimate the number and characteristics of inmates in local jails nationwide.

BJS drew a sample of local jail jurisdictions for the 2015 ASJ based on information from the 2013 Census of Jails. Local jail jurisdictions include counties (parishes in Louisiana) or municipal governments that administered one or more local jails. All jail jurisdictions were grouped into 10 strata based on their average daily population and presence of juveniles in 2013. In 8 of the 10 strata, a random sample of jail jurisdictions was selected. The remaining two strata were certainty strata, where all jurisdictions were selected with probability 1. One stratum consisted of all jails that were operated jointly by two or more jurisdictions (referred to as multijurisdictional jails). The other stratum consisted of all jail jurisdictions that—

• held juvenile inmates at the time of the 2013 Census of Jails and had an average daily population of 500 or more inmates during the 12 months ending December 31, 2013
• held only adult inmates and had an average daily population of 750 or more
• were located in California.

The ASJ sample includes all California jail jurisdictions. This sampling feature, introduced in 2013 in response to the enactment of California AB 109 and AB 117, aimed to reduce the number of inmates housed in state prisons starting on October 1, 2011. After the enactment of these two laws, the jail population in California experienced an unusual increase, which was atypical of the rest of the United States. For this reason, the ASJ sampling design was modified to include all California jail jurisdictions in a certainty (self-representing) stratum (see Methodology in Jail Inmates at Midyear 2014). The inclusion of all California jail jurisdictions resulted in an additional 21 jurisdictions.

See Methodology in each report for details specific to the collection in the Jail Inmates at Midyear series.

For methodologies prior to the 2006 ASJ, refer to the National Archive of Criminal Justice Data.

Prior to 2015, the ASJ asked jails to report the total and detailed inmate counts on the last weekday in June (the midyear reference date). In 2015 and 2016, the ASJ collected the total confined population at midyear but detailed inmate counts by characteristics (i.e., sex, race or ethnicity, age category, conviction status, and most serious type of offense) on December 31 (the year-end reference date). Starting with the 2017 collection, the ASJ reverted back to the midyear reference. Comparisons of year-end data with midyear data need to consider seasonal variations, as jails typically hold fewer inmates at year-end than at midyear.