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National Drug Threat Assessment 2005
February 2005

Appendix A

National Drug Threat Survey 2003 Methodology

The National Drug Intelligence Center (NDIC) National Drug Threat Survey 2004 (NDTS 2004) was administered to a probability-based sample of state and local law enforcement agencies. The sample was designed to provide representative data at national, regional, and state levels for use in the National Drug Threat Assessment 2005. Data from this representative sample also are used in NDIC's state and regional threat assessments.

   

Survey Instrument

The NDTS 2004 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. Responding law enforcement agencies were asked to identify the drug that poses the greatest threat, that most contributes to violent crime, and that most contributes to property crime in their areas. Agencies also were asked to rate the overall level of availability (on a scale of high, moderate, or low) of powder cocaine, crack cocaine, heroin, methamphetamine, marijuana, MDMA (ecstasy), and other dangerous drugs in their areas. The survey included an item designed to solicit information on the level of involvement of street gangs and outlaw motorcycle gangs in the distribution of drugs in general and of specific drugs. Other items in the questionnaire asked respondents to indicate the types of heroin available, predominant type of heroin, presence of crack cocaine conversion sites, presence of MDMA production laboratories, level of methamphetamine production, and nature of cannabis cultivation in their areas. Respondents also were asked to indicate which chemicals are diverted in or from their areas for the production of illicit drugs and which pharmaceuticals are commonly diverted or illicitly used in their areas.

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Sample Design

The NDTS 2004 sample used the NDTS 2003 sample with adjustments for the attrition and addition of agencies to the sampling frame as discussed below. In 2003, 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 for use in selecting law enforcement agencies to be surveyed for the NDTS 2003. 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 7,930 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 7,930 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 and county sheriff's offices with 75 or more sworn FTE employees as reported in the 2000 Census of State and Local Law Enforcement Agencies were selected with certainty (stratum 97).

  • State police and state-level investigative agencies 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 (stratum 98).

  • Investigative agencies in three U.S. territories--Guam, the Northern Mariana Islands, and Puerto Rico--were also selected with certainty (stratum 99).

To ensure that state-level representative statements could be made about results obtained from the NDTS 2003, local law enforcement agencies were coded according to the 50 states and District of Columbia. 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. The states were used as the noncertainty strata, and a Neyman allocation was used to allocate the noncertainty sample to the state strata.28 All eligible law enforcement agencies in the District of Columbia and Hawaii met the criteria for inclusion with certainty and were included in stratum 97. The state of California was split: law enforcement agencies within the Southern and Central U.S. Attorney Districts were included in Southern California and those in the Eastern and Northern Districts were included in Northern California. The noncertainty agencies in Southern California were included in stratum 91, and similar agencies for Northern California were included in stratum 92. The NDTS 2003 sample, representing the sampling universe of 7,930 agencies, consisted of 3,497 law enforcement agencies. In the course of conducting the NDTS 2003, NDIC identified nine agencies in the sample frame that were no longer eligible for inclusion in the sample frame. Prior to administering the NDTS 2004, two additional ineligible agencies were removed from the sample frame. The NDTS 2004 sample, representing the sampling universe of 7,919 state and local law enforcement agencies, consisted of 3,486 agencies in 53 strata, 3 of which were certainty strata (see Table A1).

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Data Collection

Of the 3,486 state and local law enforcement agencies in the actual sample, 498 had responded to the survey earlier in 2004 under a joint effort by NDIC and the High Intensity Drug Trafficking Area (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. NDIC verified the point of contact and mailing address for the remaining 2,988 law enforcement agencies in the sample and mailed the surveys, accompanied by a cover letter from NDIC's Director and a postage-paid return envelope. The letter also included instructions for sample agencies to complete the NDTS 2004 electronically using the secure web site, https://www.ndts.usdoj.gov, designed by NDIC and supported by the U.S. Department of Justice, Justice Data Center. Of the 2,988 agencies given the option to respond via the NDTS 2004 web site, 396 (13.3%) responded electronically. NDIC Field Program Specialists located throughout the country were responsible for follow-up contacts with sample agencies that were mailed a survey. NDIC provided daily reports to help Field Program Specialists target nonresponding agencies, which were contacted by telephone, by letter, and in person. NDIC technical support personnel assisted agencies that encountered problems when responding via the NDTS web site. All responses were entered in the NDTS database designed and developed by NDIC.

    

Sample Adjustments

During survey processing, NDIC identified three ineligible agencies that no longer performed drug enforcement activities. All three of these agencies were certainties due to size (stratum 97). A state-level investigative agency was added to stratum 98, a certainty stratum. The three ineligible agencies were deleted from the original actual sample of 3,486, and the new state-level agency was added resulting in an adjusted sample of 3,484 agencies in 53 strata, three of which were certainty strata. A poststratification factor to correct base weights in those strata was not required since none of the ineligible records were in noncertainty strata. The adjusted sample represents 7,917 agencies. A summary of the adjusted sample design is presented in Table A2.     

Nonresponse Adjustment Factor

Of the 3,484 agencies in the adjusted sample, 3,429 agencies responded to the NDTS 2004 for an overall response rate of 98.4 percent. Table A2 on page 152 summarizes the response rates by state. 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


Graphic showing the formula for calculating the nonresponse adjustment factor for each stratum j.
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

Graphic showing the formula for calculating the final weight for each responding agency.
d-link

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Estimation Techniques

The final weight for each respondent was used to derive national-, regional-, and state-level estimates for all survey items. The final adjusted score was summed for each response category (for example, high, moderate, and low) for each item, and the proportion of the final scores provided the national, regional, or state-level estimate for that item. Some respondents did not answer all survey items. The item nonresponse rate ranged from 0.3 to 19.2 percent.

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Nonsampling Error

Nonsampling error may affect NDTS 2004 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, moderate, 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 2004 data. No systemic nonsampling errors were identified.

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Table A1. NDTS 2004 Sample Design (3,429 of 3,484 Agencies Responding)

  Stratum Sample Count Total Original Base Weight Post-stratification Factor Nonresponse Adjustment Factor Final Weight
1 Alabama 54 154 2.8519 1.0000 1.0000 2.8519
2 Alaska 16 16 1.0000 1.0000 1.0000 1.0000
4 Arizona

 28 54 1.9286 1.0000 1.0000 1.9286
5 Arkansas

 54 105 1.9444 1.0000 1.0000 1.9444
8 Colorado

 22 89 4.0455 1.0000 1.0000 4.0455
9 Connecticut

 23 73 3.1739 1.0000 1.0000 3.1739
10 Delaware

 12 12 1.0000 1.0000 1.0909 1.0909
12 Florida

 39 192 4.9231 1.0000 1.0000 4.9231
13 Georgia

 49 243 4.9592 1.0000 1.0000 4.9592
16 Idaho

 50 50 1.0000 1.0000 1.0000 1.0000
17 Illinois

 76 375 4.9342 1.0000 1.0556 5.2085
18 Indiana

 55 171 3.1091 1.0000 1.0000 3.1091
19 Iowa

 58 104 1.7931 1.0000 1.0545 1.8908
20 Kansas

 46 91 1.9783 1.0000 1.0000 1.9783
21 Kentucky

 65 126 1.9385 1.0000 1.0317 2.0000
22 Louisiana

 22 109 4.9545 1.0000 1.0476 5.1903
23 Maine

 64 80 1.2500 1.0000 1.0323 1.2904
24 Maryland

 29 41 1.4138 1.0000 1.0000 1.4138
25 Massachusetts

 53 230 4.3396 1.0000 1.0600 4.6000
26 Michigan

 50 247 4.9400 1.0000 1.0204 5.0408
27 Minnesota

 63 154 2.4444 1.0000 1.2115 2.9614
28 Mississippi

 73 124 1.6986 1.0000 1.0139 1.7222
29 Missouri

 65 221 3.4000 1.0000 1.0000 3.4000
30 Montana

 32 32 1.0000 1.0000 1.0000 1.0000
31 Nebraska

 46 46 1.0000 1.0000 1.0000 1.0000
32 Nevada

 18 18 1.0000 1.0000 1.0000 1.0000
33 New Hampshire

 57 68 1.1930 1.0000 1.0556 1.2593
34 New Jersey

 73 363 4.9726 1.0000 1.0139 5.0417
35 New Mexico

 36 49 1.3611 1.0000 1.0000 1.3611
36 New York

 53 264 4.9811 1.0000 1.0000 4.9811
37 North Carolina

 51 232 4.5490 1.0000 1.0625 4.8333
38 North Dakota

 21 21 1.0000 1.0000 1.0000 1.0000
39 Ohio

 85 424 4.9882 1.0000 1.0119 5.0476
40 Oklahoma

 51 122 2.3922 1.0000 1.0000 2.3922
41 Oregon

 31 77 2.4839 1.0000 1.0000 2.4839
42 Pennsylvania

 73 360 4.9315 1.0000 1.0000 4.9315
44 Rhode Island

 26 26 1.0000 1.0000 1.0000 1.0000
45 South Carolina

 34 103 3.0294 1.0000 1.0000 3.0294
46 South Dakota

 16 16 1.0000 1.0000 1.0000 1.0000
47 Tennessee

 43 168 3.9070 1.0000 1.0000 3.9070
48 Texas

 83 414 4.9880 1.0000 1.0375 5.1751
49 Utah

 39 60 1.5385 1.0000 1.0000 1.5385
50 Vermont

 31 31 1.0000 1.0000 1.0690 1.0690
51 Virginia

 24 59 2.4583 1.0000 1.0000 2.4583
53 Washington

 42 119 2.8333 1.0000 1.0000 2.8333
54 West Virginia

 43 49 1.1395 1.0000 1.0238 1.1666
55 Wisconsin

 53 198 3.7358 1.0000 1.0000 3.7358
56 Wyoming

 28 28 1.0000 1.0000 1.0000 1.0000
91 Southern California

 11 58 5.2727 1.0000 1.0000 5.2727
92 Northern California

 34 167 4.9118 1.0000 1.0000 4.9118
97 Certainties due to size
(75 or more FTEs)

 1209 1209 1.0000 1.0000 1.0100 1.0100
98 State agency certainties 72 72 1.0000 1.0000 1.0000 1.0000
99 Certainty agencies outside United States

 3 3 1.0000 1.0000 1.0000 1.0000

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Table A2. NDTS 2003 Response Rates    

State/Territory/District Respondents Sample Size Response Rate (%)
Guam, Northern Mariana Islands, Puerto Rico    3     3 100.0
Alabama  75   75 100.0
Alaska   18   18 100.0
Arizona   47   47 100.0
Arkansas   69   69 100.0
California 184 184 100.0
Colorado   48   48 100.0
Connecticut   47   48   97.9
Delaware   15   16   93.8
District of Columbia     1     1 100.0
Florida 138 138 100.0
Georgia   96   96 100.0
Hawaii     5     5 100.0
Idaho   57   57 100.0
Illinois 119 123   96.7
Indiana   82   82 100.0
Iowa   68   71   95.8
Kansas   60   60 100.0
Kentucky   69   71   97.2
Louisiana   64   65   98.5
Maine   66   68   97.1
Maryland   47   47 100.0
Massachusetts   90   93   96.8
Michigan   88   89   98.9
Minnesota   66   79   83.5
Mississippi   86   87   98.9
Missouri   89   89 100.0
Montana   37   37 100.0
Nebraska   51   51 100.0
Nevada   28   28 100.0
New Hampshire   59   62   95.2
New Jersey 137 142   96.5
New Mexico   48   48 100.0
New York 107 107 100.0
North Carolina 102 106   96.2
North Dakota   25   25 100.0
Ohio 127 128   99.2
Oklahoma   64   64 100.0
Oregon   50   50 100.0
Pennsylvania   92   92 100.0
Rhode Island   35   35 100.0
South Carolina   62   63   98.4
South Dakota   20   20 100.0
Tennessee   72   72 100.0
Texas 158 164   96.3
Utah   50   50 100.0
Vermont   31   33   93.9
Virginia   48   48 100.0
Washington   63   63 100.0
West Virginia   47   48   97.9
Wisconsin   85   85 100.0
Wyoming   34   34 100.0
 

End Note

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

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