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.

The Bureau of Justice Statistics drew a sample of local jail jurisdictions for the 2015 Annual Survey of Jails, 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 was 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 (NCJ 248629, BJS web, June 2015). 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.

The questionnaire is modified periodically to capture information on evolving issues in criminal justice and topics of the greatest interest to users. Future modifications include enhancing our data collection instrument in an effort to address issues related to admissions and releases of inmates; inmate length of stay in jail; jail capacity and crowding; staff workload; and medical, mental health, and substance abuse treatment issues.