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Appendix A: National Drug Threat Survey 2002 Methodology

NDIC's National Drug Threat Survey (NDTS) 2002 was administered to a probability-based sample of state and local law enforcement agencies. The sample was designed to provide nationally and regionally representative data for use in the National Drug Threat Assessment 2003. Previous versions of the NDTS, conducted in 2000 and 2001, were targeted research projects based on nonprobability, purposive sampling plans that limited the generalization of results to only those agencies responding in a given year.   Survey Instrument The NDTS 2002 questionnaire (OMB Number 1105-0071) was designed by NDIC. A thorough review of data and response patterns from previous versions of the NDTS was conducted to improve the accuracy of information obtained from respondents. Items in the survey were designed to assess the availability, abuse, overall threat, and past year change in the overall threat of all major drugs of abuse in the United States. Responding law enforcement agencies were asked to rank the greatest drug threats in their areas and to identify not only source cities for the powder cocaine, crack cocaine, heroin, methamphetamine, marijuana, and MDMA transported to their areas but also destination cities for these drugs transported out of their areas. The survey also solicited information on specific groups involved in the transportation, wholesale distribution, and retail sales of these drugs. Information on cultivation, production, and chemical procurement was solicited when applicable to specific drugs. The survey contained open-ended items that permitted responding agencies to provide detailed qualitative information on various aspects of the overall drug situation and the threat of specific drugs in their areas. Such information was used to substantiate and expand drug threat information obtained from other federal, state, and local law enforcement reporting used in the preparation of the National Drug Threat Assessment 2003. To Top      To Contents   Sample Design The 2000 Census of State and Local Law Enforcement Agencies conducted by the U.S. Bureau of Justice Statistics was the basis for determining a sample frame from which to select law enforcement agencies to be surveyed for the NDTS 2002. After careful review of the more than 17,000 law enforcement agencies in the 2000 Census of State and Local Law Enforcement Agencies, a final sample frame of 7,932 state and local law enforcement agencies with drug law enforcement responsibilities was created. Municipal police departments from every state, including regional and county police departments with 10 or more sworn full time equivalent (FTE) employees, were retained for the sampling frame. County sheriff's offices with 10 or more sworn FTE employees were also retained for the sampling frame except those in six states where county sheriff's offices do not have drug law enforcement responsibilities. In the rest of the country, sheriff's offices were excluded if they did not indicate on the 2000 Census of State and Local Law Enforcement Agencies that they enforce drug laws. Campus police departments, constables, and special police agencies were excluded since most of these agencies, too, have limited or no drug investigation responsibilities. Tribal police departments, whose jurisdictions fall under federal authority, also were eliminated. State drug investigative agencies not in the 2000 Census of State and Local Law Enforcement Agencies were added to the sampling universe. The sample frame of 7,932 state and local law enforcement agencies was stratified to include the following specific groups of state and local law enforcement agencies to ensure a thorough analysis of the domestic drug situation. Municipal police departments (stratum 50) or county sheriff's offices (stratum 40) with 75 or more sworn FTE employees as reported in the 2000 Census of State and Local Law Enforcement Agencies were selected with certainty. State police and state-level investigative agencies (stratum 10) were selected with certainty to provide information on the drug threat situation from a state perspective. State police agencies were obtained from the 2000 Census of State and Local Law Enforcement Agencies. Additional state-level investigative agencies were derived from previous NDTS sampling plans. Typically included for each state were the state police and lead drug enforcement agency, although this pattern varied in some states. All U.S. members of the Major Cities Chiefs Association (stratum 20) were selected with certainty to ensure adequate assessment of the drug threat situation in major cities, which are the location of many of the primary drug markets in the United States. These agencies were in the 2000 Census of State and Local Law Enforcement Agencies and were identified in a stratum to enable detailed analysis of responses from these agencies by NDIC. A final stratum selected with certainty comprises 26 municipal police departments and county sheriff's offices with jurisdictions that correspond to ports of entry located along the U.S.-Mexico border (stratum 30). These agencies were in the 2000 Census of State and Local Law Enforcement Agencies and were selected with certainty to allow for analysis of transportation and distribution information provided by agencies at these critical drug trafficking areas. To ensure that regionally representative statements could be made about results obtained from the NDTS 2002, local law enforcement agencies were coded according to the nine OCDETF regions. Municipal police departments and county sheriff's offices with sworn FTE employees of 10 or more but fewer than 75, and meeting all the criteria discussed above, were included in these strata. These OCDETF regions were used as the noncertainty strata, and a Neyman allocation21  was used to allocate the noncertainty sample to the OCDETF region strata. The actual sample, representing the sampling universe of 7,932 state and local law enforcement agencies, consisted of 3,002 agencies in 23 strata, 5 of which were certainty strata. A summary of the sample design is presented in Table A1. To Top      To Contents   Data Collection NDIC verified the point of contact and mailing address for each law enforcement agency in the sample and mailed the surveys, which were accompanied by a cover letter from NDIC Director Michael T. Horn and a postage-paid return envelope. Field Program Specialists located throughout the country were responsible for follow-up contacts with sample agencies that were mailed a survey. Of the 3,002 state and local law enforcement agencies in the actual sample, 383 had received the survey earlier in 2002 under a joint effort by NDIC and the HIDTA program that was designed to assist the HIDTAs in preparing their annual threat assessments. Copies of surveys completed by sample agencies under the joint NDIC-HIDTA effort were forwarded to NDIC. Lists of agencies that did not respond were given to Field Program Specialists for follow-up contact, and a second NDTS 2002 survey was either mailed or personally delivered to the nonresponding agency. NDIC provided daily reports to help Field Program Specialists target nonresponding agencies, which were contacted by telephone, by letter, and in person. All responses were entered in the NDTS database designed and developed by NDIC.   Sample Adjustments During the survey process, NDIC identified 21 state police and investigative agency records in the original sample as duplicates. Duplicate records were removed, modifying the number of state police and investigative agencies in the original sample file from 99 to 78. NDIC also identified five agencies as ineligible because they had merged with other law enforcement agencies during the period of administering the survey. Two of these agencies were large county sheriff's offices (stratum 40), one was a large county police department (stratum 50), one was a small county police department in the Florida/Caribbean OCDETF region (stratum 55), and one was a small county police department in the Great Lakes OCDETF region (stratum 56). The original actual sample of 3,002 was corrected to an adjusted sample of 2,976 agencies in 23 strata, 5 of which were certainty strata. To compensate for the deletion of the two ineligible records in noncertainty strata (stratum 55 and stratum 56) from the sample, a poststratification factor was calculated for the affected strata to correct the base weights for those strata. For all other strata, the poststratification factor is 1.0. The poststratification factors for all strata also are shown in Table 1. To Top      To Contents   Nonresponse Adjustment Factor Of the 2,976 agencies in the adjusted sample, 2,386 agencies responded to the NDTS 2002 for an overall response rate of 80.17 percent. Table 2 summarizes the response rates by OCDETF region. A nonresponse adjustment factor was applied to account for those agencies that did not respond to the survey. The nonresponse adjustment factor for each stratum j is calculated as d-link where k represents either the kth responding or the kth nonresponding agency in stratum j. The final weight for each responding agency is calculated as . d-link   Estimation Techniques The final weight for each respondent was used to derive national and regional estimates for all applicable survey items (nominal and ordinal data questions). The final adjusted score was summed for each response category (for example, high, medium, and low) for each item, and the proportion of the final scores provided the national or regional estimate for that item. Some respondents did not answer all survey items. The item nonresponse rate ranged from 1.2 to 5.5 percent.   Nonsampling Error Nonsampling error may affect NDTS 2002 data. Possible nonsampling errors include the following: Inability to obtain information about all agencies in the sample Varied interpretation of response categories (for example, high, medium, and low are defined differently by respondents) Inability or unwillingness of respondents to provide correct information Errors made in collection, coding, or processing of data Failure to represent all agencies within the sample (undercoverage) Nonsampling error can increase the total error over the error resulting from sampling. Random nonsampling errors can increase the variability of data, while systemic nonsampling errors that are consistent in one direction can introduce bias into the results of a sample survey. NDIC used data collection, coding, and processing procedures designed to limit the effects of random nonsampling error on the NDTS 2002 data. No systemic nonsampling errors were identified.

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End Note

21. For more details on Neyman allocation, W.G. Cochran, "Stratified Random Sampling," Chapter 5 in Sampling Techniques, 3d ed. New York: John Wiley and Sons, 1977.

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