Sourcebook of Criminal Justice Statistics

Appendix 16

National Jail Census, Annual Survey of
Jails, and Survey of Inmates in Local Jails

Methodology and survey sampling procedures

Note: The following information was excerpted from the U.S. Department of Justice, Bureau of Justice Statistics, Jails and Jail Inmates 1993-94, Bulletin NCJ-151651 (Washington, DC: U.S. Department of Justice, April 1995), pp. 14-16; Prison and Jail Inmates at Midyear 1997, Bulletin NCJ-167247 (Washington, DC: U.S. Department of Justice, January 1998), pp. 9, 10; Profile of Jail Inmates, 1996, Special Report NCJ-164620 (Washington, DC: U.S. Department of Justice, April 1998), pp. 13, 14; and information provided by the U.S. Department of Justice, Bureau of Justice Statistics. Non-substantive editorial adaptations have been made.

Methodology

National Jail Census

The National Jail Census is taken every 5 years and is conducted for the U.S. Department of Justice, Bureau of Justice Statistics (BJS) by the U.S. Bureau of the Census. Data are presented for censuses conducted in 1983, 1988, and 1993. For 1983, questionnaires were mailed to all (3,358) locally administered jails in the Nation. For 1988, questionnaires were mailed to 3,448 locally administered jails. Forty-four jails were added to the initial mailing and 176 were deleted, yielding a final count of 3,316 facilities for 1988. Through the use of various followup procedures, a response rate of 99% was achieved for 1983, and 100% for 1988.

The most recent census, conducted in 1993, included all locally administered confinement facilities (3,287) that hold inmates beyond arraignment and are staffed by municipal or county employees. The census also included 17 jails that were privately operated under contract for local governments and 7 facilities maintained by the Federal Bureau of Prisons and functioning as jails.

Excluded from the census were temporary holding facilities, such as drunk tanks and police lockups, that do not hold persons after being formally charged in court (usually within 72 hours of arrest). Also excluded were State-operated facilities in Alaska, Connecticut, Delaware, Hawaii, Rhode Island, and Vermont, which have combined jail-prison systems. Five locally operated jails in Alaska were included.

The mailing list used for the census was derived from the National Justice Agency List maintained by the U.S. Bureau of the Census for BJS. The census forms were mailed to 3,506 facilities on June 22, 1993. Twenty-eight jails were added to the initial mailing and 230 were deleted, resulting in a total of 3,304 facilities.

After extensive followup, including additional mail requests and repeated telephone contacts, all jails (except those in one jurisdiction) provided data for four critical data items--number of inmates, average daily population, rated capacity, and sex of inmates housed. Data on these four items for the nonresponding jails were imputed based on data reported in the 1994 Annual Survey of Jails.

Completed forms with data for all or most items were received for 2,981 jails, resulting in a 90% response rate. These reporting jails housed 93% of all local jail inmates on June 30, 1993. Because there was nonresponse and incomplete data on all census items except the four critical items, national totals had to be estimated. Readers interested in the estimation procedures employed should consult the original Source (Source, Jails and Jail Inmates 1993-94).

Annual Survey of Jails

In each of the 4 years between the full censuses, a survey of jails is conducted to estimate baseline characteristics of the Nation's jails and inmates housed in the jails. Data from the Annual Survey of Jails are presented for 1984-87, 1989-92, and 1994-97. The reference date for each of these surveys was June 30, except 1990 when the reference date was June 29, and 1991 and 1996 when it was June 28. All surveys prior to the 1994 survey were based on all jails in jurisdictions with 100 or more jail inmates and a stratified random sample of jurisdictions with an average daily population of less than 100 inmates. For 1984, 1,164 jails in 893 jurisdictions were included; in 1985, 1,142 jails in 874 jurisdictions were included; in 1986, 1,137 jails in 868 jurisdictions were included; in 1987, 1,135 jails in 866 jurisdictions were included; in 1989, 1,128 jails in 809 jurisdictions were included; in 1990, 1,135 jails in 804 jurisdictions were included; in 1991, 1,124 jails in 799 jurisdictions were included; and in 1992, 1,113 jails in 795 jurisdictions were included.

A new sample of jail jurisdictions was selected for the 1994-97 surveys using information from the 1993 census. A jurisdiction is a county (parish in Louisiana) or municipal government that administers one or more local jails. The 1997 sample included all jails in 795 selected jail jurisdictions and 25 multijurisdiction jails. A multijurisdiction jail is one in which two or more jurisdictions have a formal agreement to operate the facility.

In drawing the samples for 1994-97, jail jurisdictions were first stratified into two groups: single jurisdiction jails and multijurisdiction jails. All of the multijurisdiction jails were included in the survey. The remaining jurisdictions were then further stratified into two groups: jurisdictions with jails authorized to hold juveniles and jurisdictions with jails holding only adults. Jurisdictions were then selected based on the average daily population in the 1993 census. In 1997, all jails in 204 jurisdictions were automatically included if the jurisdiction held juveniles and had an average daily population of 250 or more inmates in 1993 or if they held only adults and had an average population of 500 or more. The other jurisdictions (591) were then selected based on stratified probability sampling.

Data were obtained by mailed questionnaires. After followup phone calls to nonrespondents, the response rate for the 1997 survey was 100%.

Sampling error

Survey estimates have an associated sampling error because jurisdictions with smaller average daily populations were sampled for the survey. Estimates based on the sample survey may differ somewhat from the results of conducting a complete census. Different samples could yield somewhat different results. Standard error is a measure of the variation among the estimates from all possible samples, stating the precision with which an estimate from a particular sample approximates the average of all possible samples. The estimated relative sampling error for the total number of persons under the jurisdiction of jail authorities of 637,319 on June 30, 1997, was 0.86%; for persons held in the custody of jail authorities of 567,079, was 0.64%. Readers interested in standard error estimates should consult the original Source (Source, Prison and Jail Inmates at Midyear 1997).

Measures of population

Two measures of inmate population are used: the average daily population for the year ending June 30 and the inmate count on June 30 of each year. The average daily population balances out any extraordinary events that may render atypical the inmate count on June 30. The June 30 count provides data on characteristics of inmates, such as race, Hispanic origin, and age, that may not be available on an annual basis.

For the first time, in 1995 the Annual Survey of Jails obtained separate counts of the total number of offenders under jail jurisdiction, those held in jail facilities, and those supervised outside of jail facilities. Previous surveys and censuses included a small but unknown number of offenders under community supervision. To estimate the percent change from 1994 to 1995 in the jail population, the 1995 survey included a count of inmates held at midyear 1994.

In the 1996 survey the number of persons supervised outside a jail facility included for the first time persons under drug, alcohol, mental health, or other medical treatment. Comparison with 1995 estimates should exclude these persons.

Juveniles

State statutes and judicial practices allow juveniles to be incarcerated in adult jails under a variety of circumstances. Because of differing statutes and practices, however, accurate and comparable data on juveniles are difficult to collect. Beginning in 1994, the Annual Survey of Jails provided estimates of the total number of jail inmates under age 18, the number held as adults, and the number held as juveniles. New sampling procedures also were introduced in 1994 to minimize the standard errors of these estimates. By stratifying jurisdictions based on the authority to house juveniles, the precision of the juvenile counts was improved.

Survey of Inmates in Local Jails

The 1996 Survey of Inmates in Local Jails was conducted for BJS by the U.S. Bureau of the Census. Similar surveys of jail inmates were conducted in 1972, 1978, 1983, and 1989. Interviews for the 1996 survey were conducted from October 1995 through March 1996.

Interviews were about an hour long and used computer-assisted personal interviewing (CAPI). With CAPI, computers provide the interviewer questions, including followup questions tailored to preceding answers. Before the interview inmates were told verbally and in writing that participation was voluntary and that all information provided would be held in confidence. Participants were assured that the survey was solely for statistical purposes and that no individual could be identified through use of survey results.

Sample design

The sample for the 1996 survey was selected from a universe of 3,328 jails that were enumerated in the 1993 Census of Jails or opened after the Census but before the spring of 1996. The sample design was stratified two-stage selection. In the first stage six separate strata were formed based on the size of the male and female populations. In two strata all jails were selected--those jails housing only females and those with more than 1,000 males or more than 50 females or both. In the remaining four strata a systematic sample of jails was selected. Each jail within a stratum had an equal probability of selection. Equal probabilities were used instead of probability proportional to size because jail populations were likely to change between 1993 and 1995. Overall, 462 jails were selected. Interviews were conducted in 431; 19 refused, 8 were closed, and 4 were on the universe list in error.

In the second sampling stage, interviewers visited each selected facility and systematically selected a sample of male and female inmates using predetermined procedures. As a result, approximately 1 in every 100 males were selected in 4 strata and 1 in 83 in the male stratum. Depending on the stratum, 1 in 50, 25, 24, or 21 females were selected. A total of 6,133 inmates were interviewed, and 738 refused to participate, for a second stage nonresponse of 10.8%. The total nonresponse from both stages was 13.7%.

Based on the completed interviews, estimates for the entire population were developed using weighting factors derived from the original probability of selection in the sample. These factors were adjusted for variable rates of nonresponse across strata and inmate characteristics. Further adjustments were made to control the survey estimates to counts of jail inmates obtained from the 1993 Census of Jails and the 1995 Annual Survey of Jails.

Accuracy of the survey estimates

The accuracy of the estimates from the 1996 Survey of Inmates in Local Jails depends on two types of error: sampling and measurement. Sampling error is variation that may occur by chance because a sample rather than a complete enumeration of the population was conducted. Measurement error can be attributed to many sources, such as nonresponse, differences in the interpretation of questions among inmates, recall difficulties, and processing errors. In any survey the full extent of the measurement error is never known.

Measurement changes

Conviction status--In the 1996 survey, inmates still serving sentence for an offense--that is, on probation, parole, or other conditional release--when most recently admitted to jail were classified as sentenced. In prior surveys, status was based on the offense for which they were most recently admitted to jail. For the 1996 survey 55.2% were classified as sentenced, but that percentage would have been 45.9% under the classification of previous surveys.

Educational attainment--Starting with the 1996 survey, inmates who had not finished high school were asked if they had earned a GED or high school equivalency certificate, which was classified as a high school diploma. This change raised the percentage of high school graduates from 25.9% to 30.0%. GED responses were volunteered by respondents in prior surveys.