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2001 State Estimates of Substance Use

bulletNational data      bulletState level data       bulletMetropolitan and other subState area data

Appendix G: Statistical Methods and Limitations of the Data

G.1. Target Population

An important limitation of the National Household Survey on Drug Abuse (NHSDA) estimates of drug use prevalence is that they are only designed to describe the target population of the survey-the civilian, noninstitutionalized population aged 12 or older. Although this population includes almost 98 percent of the total U.S. population age 12 or older, it excludes some important and unique subpopulations who may have very different drug-using patterns. For example, the survey excludes active military personnel, who have been shown to have significantly lower rates of illicit drug use. Persons living in institutional group quarters, such as prisons and residential drug treatment centers, are not included in the NHSDA and have been shown in other surveys to have higher rates of illicit drug use. Also excluded are homeless persons not living in a shelter on the survey date, another population shown to have higher than average rates of illicit drug use. Appendix H describes other surveys that provide data for these populations.

G.2. Nonsampling Error

Nonsampling errors can occur from nonresponse, coding errors, computer processing errors, errors in the sampling frame, reporting errors, and other errors not due to sampling. Nonsampling errors are reduced through data editing, statistical adjustments for nonresponse, close monitoring and periodic retraining of interviewers, and improvement in various quality control procedures.

Although nonsampling errors can often be much larger than sampling errors, measurement of most nonsampling errors is difficult or impossible. However, some indication of the effects of some types of nonsampling errors can be obtained through proxy measures, such as response rates and from other research studies.

G.2.1 Screening and Interview Response Rate Patterns

Response rates for the NHSDA were stable for the period from 1994 to 1998, with the screening response rate at about 93 percent and the interview response rate at about 78 percent (response rates discussed in this appendix are weighted). In 1999, the computer-assisted interviewing (CAI) screening response rate was 89.6 percent, and the interview response rate was 68.6 percent. A more stable and experienced field interviewer (FI) workforce improved these rates in 2000 and continued in 2001. Of the 171,519 eligible households sampled for the 2001 NHSDA main study, 157,471 were successfully screened for a weighted screening response rate of 91.9 percent (Table G.1). In these screened households, a total of 89,745 sample persons were selected, and completed interviews were obtained from 68,929 of these sample persons, for a weighted interview response rate of 73.3 percent (see Table G.5). A total of 13,478 (16.5 percent) sample persons were classified as refusals or parental refusals, 4,681 (5.3 percent) were not available or never at home, and 2,657 (4.9 percent) did not participate for various other reasons, such as physical or mental incompetence or language barrier (Table G.2). Table s G.3 and G.4 show the distribution of the selected sample by interview code and age group. The weighted interview response rate was highest among 12 to 17 year olds (82.2 percent), females (74.6 percent), blacks and Hispanics (75.0 and 78.8 percent, respectively), in nonmetropolitan areas (76.7 percent), and among persons residing in the South (74.4 percent) (Table G.5).

The overall weighted response rate, defined as the product of the weighted screening response rate and weighted interview response rate, was 61.5 percent in 1999, 68.6 percent in 2000, and 67.3 percent in 2001. Nonresponse bias can be expressed as the product of the nonresponse rate (1-R) and the difference between the characteristic of interest between respondents and nonrespondents in the population (Pr - Pnr). Thus, assuming the quantity (Pr - Pnr) is fixed over time, the improvement in response rates in 2000 and 2001 over 1999 will result in estimates with lower nonresponse bias.

G.2.2 Inconsistent Responses and Item Nonresponse

Among survey participants, item response rates were above 97 percent for most questionnaire items. However, inconsistent responses for some items, including the drug use items, were common. Estimates of substance use from the NHSDA are based on the responses to multiple questions by respondents, so that the maximum amount of information is used in determining whether a respondent is classified as a drug user. Inconsistencies in responses are resolved through a logical editing process that involves some judgment on the part of survey analysts and is a potential source of nonsampling error. Because of the automatic routing through the CAI questionnaire (e.g., lifetime drug use questions that skip entire modules when answered "no"), there is less editing of this type than in the paper-and-pencil interviewing (PAPI) questionnaire used prior to the NHSDA redesign in 1999.

In addition, logical editing is used less often because with the CAI data, statistical imputation is relied upon more heavily to determine the final values of drug use variables in cases where there is the potential to use logical editing to make a determination. The combined amount of editing and imputation in the CAI data is still considerably less than the total amount used in prior PAPI surveys. For the 2001 CAI data, for example, 6.7 percent of the estimate of past month hallucinogen use was based on logically edited cases and 6.6 percent on imputed cases, for a combined amount of 13.3 percent. In the 1998 NHSDA (administered using PAPI), the amount of editing and imputation for past month hallucinogen use was 60.3 and 0.0 percent, respectively, for a total of 60.3 percent. The combined amount of editing and imputation for the estimate of past month heroin use was 5.7 percent for the 2001 CAI and 37.0 percent for the 1998 PAPI data.

G.2.3 Validity of Self-Reported Use

NHSDA estimates are based on self-reports of drug use, and their value depends on respondents' truthfulness and memory. Although many studies have generally established the validity of self-report data and the NHSDA procedures were designed to encourage honesty and recall, some degree of underreporting is assumed (Harrell, 1997; Harrison & Hughes, 1997; Rouse, Kozel, & Richards, 1985). No adjustment to NHSDA data is made to correct for this. The methodology used in the NHSDA has been shown to produce more valid results than other self-report methods (e.g., by telephone) (Aquilino, 1994; Turner, Lessler, & Gfroerer, 1992). However, comparisons of NHSDA data with data from surveys conducted in classrooms suggest that underreporting of drug use by youths in their homes may be substantial (Gfroerer, 1993; Gfroerer, Wright, & Kopstein, 1997).

G.3. Incidence Estimates

The average annual numbers of marijuana initiates and rates by State were obtained using small area estimation (SAE) methods applied to the pooled 2000–2001 survey data and are, therefore, different from incidence estimates reported in the other reports. NHSDA State estimates of each substance use measure are produced by combining an estimate of the measure based on the State sample data with the estimate of the measure based on a national regression model applied to local-area county and Census block group/tract-level estimates from the State. The parameters of the regression model are estimated from the entire national sample. Because the 42 smaller (in terms of population) States and the District of Columbia have smaller samples than the eight large States, estimates for the smaller States rely more heavily on the national model. The model for each substance use measure typically utilizes from 50 to 100 independent variables in the estimation. These variables include basic demographic characteristics of respondents (e.g., age, race/ethnicity, and gender), demographic and socioeconomic characteristics of the Census tract or block group (e.g., average family income and percentage of single-mother households), and county-level substance abuse and other indicators (e.g., rate of substance abuse treatment, drug arrest rate, and drug- and alcohol-related mortality rate). Population counts by State and age group are applied to the estimated rates to obtain the estimated number of persons with the substance use characteristic. Corresponding to each SAE estimate is a 95 percent prediction interval (PI) that indicates the precision of the estimate. The PI accounts for variation due to sampling, as well as variation due to the model, and is derived from the process that generates the State SAE. There is a 95 percent probability that the true value lies within the interval.

The incidence estimates discussed in this report are based on the combination of two separate SAE measures, calculated from the pooled 2000–2001 data:

Each of these measures is generated independently using SAE, by State and age group. The following formula was used to generate the average annual rate of first use of marijuana for each State:

Average annual incidence rate ={(Number of marijuana initiates in past 24 months)/
[(Number of marijuana initiates in past 24 months * 0.5) +
Number of persons who never used marijuana]}/2

For diseases, the incidence rate for a population, IR, is defined as the number of new cases of the disease, N, divided by the person time, PT, of exposure (i.e., IR = N / PT). The person time of exposure can be measured for the full period of the study or for a shorter period. The person time of exposure ends at the time of diagnosis (e.g., Greenberg, Daniels, Flanders, Eley, & Boring, 1996, pp. 16–19). Similar conventions are applied for defining the incidence of first use of a substance.

Beginning in 1999, the NHSDA questionnaire allows for collection of year and month of first use for recent initiates. Month, day, and year of birth also are obtained directly or imputed in the process. In addition, the questionnaire call record provides the date of the interview. By imputing a day of first use within the year and month of first use reported or imputed, the key respondent inputs in terms of exact dates are known. Using these respondent inputs, one can determine whether a person's first use episode occurred in the 24 months prior to the interview.

With person time of exposure measured in terms of 2-year units of time, the correct multiplier for the number of initiates in the past 24 months in the denominator of the SAE-based Average annual incidence rate is the average fraction of the exposure interval experienced prior to the initiation. Direct survey estimates of this average fraction of exposure experience prior to the initiation could be formed for each State-by-age-group combination, but direct estimates would be too imprecise to include in the SAE incidence rate estimation. Instead, the average fraction of exposure among initiates was assumed to be ½ of the 2-year exposure period. This approximation follows from the assumption that initiation episodes are distributed uniformly over the 2-year exposure period. Note that the "never" users at interview were all exposed for the full 2-year initiation period. The 24-month SAE incidence rates were then transformed into average 12-month or average annual rates by the ½ multiplier. Alternatively, one can view the final multiplication by ½ as transforming the person time units of exposure in the denominator of the rate from the number of 2-year exposure units to the number of person years of exposure.

G.4. Serious Mental Illness Estimates

For the 2001 NHSDA, mental health among adults was measured using a scale to ascertain serious mental illness (SMI). This scale consisted of six questions that ask respondents how frequently they experienced symptoms of psychological distress during the 1 month in the past year when they were at their worst emotionally. The use of this scale is based on a methodological study designed to evaluate several screening scales for measuring SMI in the NHSDA. These scales consisted of a truncated version of the World Health Organization (WHO) Composite International Diagnostic Interview Short Form (CIDI-SF) scale (Kessler, Andrews, Mroczek, Üstün, & Wittchen, 1998), the K10/K6 scale of nonspecific psychological distress (Furukawa, Kessler, Slade, & Andrews, 2003), and the WHO Disability Assessment Schedule (WHO-DAS) (Rehm et al., 1999).

The methodological study to evaluate the scales consisted of 155 respondents selected from a first-stage sample of 1,000 adults age 18 or older. First-stage respondents were selected from the Boston metropolitan area and screened on the telephone to determine whether they had any emotional problems. Respondents reporting emotional problems at the first stage were oversampled when selecting the 155 respondents at the second stage. The selected respondents were interviewed by trained clinicians in their home using both the NHSDA methodology and a structured clinical interview. The first interview included the three scales described above using audio computer-assisted self-interviewing (ACASI). Respondents completed the ACASI portion of the interview without discussing their answers with the clinician. After completing the ACASI interview, respondents were then interviewed using the 12-month nonpatient version of the Structured Clinical Interview for DSM-IV (SCID) (First, Spitzer, Gibbon, & Williams, 1997) and the Global Assessment of Functioning (GAF) (Endicott, Spitzer, Fleiss, & Cohen, 1976) to classify respondents as either having or not having SMI.

The data from the 155 respondents were analyzed using logistic regression analysis to predict SMI from the scores on the screening questions. Analysis of the model fit indicated that each of the scales alone and in combination were significant predictors of SMI and the best fitting models contained either the CIDI-SF or the K6/K10 alone. Receiver operating characteristic (ROC) curve analysis was used to evaluate the precision of the scales to discriminate between respondents with and without SMI. This analysis indicated that the K6 was the best predictor. The results of the methodological study are described in more detail in Kessler et al. (2002, 2003).

To score the items on the K6 scales, they were first coded from 0 to 4 and summed to yield a number between 0 and 24. This involved transforming response categories for the six questions (DSNERV1, DSHOPE, DSFIDG, DSNOCHR, DSEFFORT, and DSDOWN) given below so that "all of the time" is coded 4, "most of the time" is coded 3, "some of the time" 2, "a little of the time" 1, and "none of the time" 0, with "don't know" and "refuse" also coded 0. Summing across the transformed responses obtains a score with a range from 0 to 24. Respondents with a total score of 13 or greater were classified as having a past year SMI. This cutpoint was chosen to equalize false positives and false negatives.

The questions comprising the K6 scale are given below:

DSNERV1

Most people have periods when they are not at their best emotionally. Think of one month in the past 12 months when you were the most depressed, anxious, or emotionally stressed. If there was no month like this, think of a typical month.

During that month, how often did you feel nervous?

1     All of the time
2     Most of the time
3     Some of the time
4     A little of the time
5     None of the time
DK/REF

Response categories are the same for the following questions:

DSHOPE During that same month when you were at your worst emotionally . . . how often did you feel hopeless?
 
DSFIDG During that same month when you were at your worst emotionally . . . how often did you feel restless or fidgety?
 
DSNOCHR During that same month when you were at your worst emotionally . . . how often did you feel so sad or depressed that nothing could cheer you up?
 
DSEFFORT During that same month when you were at your worst emotionally . . . how often did you feel that everything was an effort?
 
DSDOWN During that same month when you were at your worst emotionally . . . how often did you feel down on yourself, no good, or worthless?

G.5. References

Aquilino, W. S. (1994). Interview mode effects in surveys of drug and alcohol use: A field experiment. Public Opinion Quarterly, 58, 210–240.

Endicott, J., Spitzer, R. L., Fleiss, J. L., & Cohen, J. (1976). The Global Assessment Scale: A procedure for measuring overall severity of psychiatric disturbance. Archives of General Psychiatry, 33, 766–771.

First, M. B., Spitzer, R. L., Gibbon, M., & Williams, J. B. W. (1997). Structured Clinical Interview for DSM-IV Axis I Disorders, Research Version, Non-patient Edition (SCID-I/NP). New York: New York State Psychiatric Institute, Biometrics Research.

Furukawa, T. A., Kessler, R. C., Slade, T., & Andrews, G. (2003). The performance of the K6 and K10 screening scales for psychological distress in the Australian National Survey of Mental Health and Well-Being. Psychological Medicine, 33, 357–362.

Gfroerer, J. (1993). An overview of the National Household Survey on Drug Abuse and related methodological research. In Proceedings of the Survey Research Section of the American Statistical Association, Joint Statistical Meetings, Boston, Massachusetts, August 1992 (pp. 464–469). Alexandria, VA: American Statistical Association.

Gfroerer, J., Wright, D., & Kopstein, A. (1997). Prevalence of youth substance use: The impact of methodological differences between two national surveys. Drug and Alcohol Dependence, 47, 19–30.

Greenberg, R. S., Daniels, S. R., Flanders, W. D., Eley, J. W., & Boring, J. R. (1996). Medical epidemiology. Norwalk, CT: Appleton & Lange.

Harrell, A. V. (1997). The validity of self-reported drug use data: The accuracy of responses on confidential self-administered answer sheets. In L. Harrison & A. Hughes (Eds.), The validity of self-reported drug use: Improving the accuracy of survey estimates (NIH Publication No. 97–4147, NIDA Research Monograph 167, pp. 37–58). Rockville, MD: National Institute on Drug Abuse.

Harrison, L., & Hughes, A. (Eds.). (1997). The validity of self-reported drug use: Improving the accuracy of survey estimates (NIH Publication No. 97–4147, NIDA Research Monograph 167). Rockville, MD: National Institute on Drug Abuse.

Kessler, R. C., Andrews, G., Colpe, L. J., Hiripi, E., Mroczek, D. K., Normand, S.-L., Walters, E. E., & Zaslavsky, A. (2002). Short screening scales to monitor population prevalences and trends in non-specific psychological distress. Psychological Medicine, 32, 959–976.

Kessler, R. C., Andrews, G., Mroczek, D., Üstün, T. B., & Wittchen, H.-U. (1998). The World Health Organization Composite International Diagnostic Interview Short Form (CIDI-SF). International Journal of Methods in Psychiatric Research, 7, 171–185.

Kessler, R. C., Barker, P. R., Colpe, L. J., Epstein, J. F., Gfroerer, J. C., Hiripi, E., Howes, M. J., Normand, S.-L. T., Manderscheid, R. W., Walters, E. E., & Zaslavsky, A. M. (2003). Screening for serious mental illness in the general population. Archives of General Psychiatry, 60, 184–189.

Rehm, J., Üstün, T. B., Saxena, S., Nelson, C. B., Chatterji, S., Ivis, F., & Adlaf, E. (1999). On the development and psychometric testing of the WHO screening instrument to assess disablement in the general population. International Journal of Methods in Psychiatric Research, 8, 110–123.

Rouse, B. A., Kozel, N. J., & Richards, L. G. (Eds.). (1985). Self-report methods of estimating drug use: Meeting current challenges to validity (DHHS Publication No.ADM 85–1402, NIDA Research Monograph 57). Rockville, MD: National Institute on Drug Abuse.

Turner, C. F., Lessler, J. T., & Gfroerer, J. C. (Eds.). (1992). Survey measurement of drug use: Methodological studies (DHHS Publication No. ADM 92–1929). Rockville, MD: National Institute on Drug Abuse.

Table G.1 Weighted Percentages and Sample Sizes for 1999 to 2001 NHSDAs, by Screening Result Code
Screening Result 1999 NHSDA 2000 NHSDA 2001 NHSDA
Sample
Size
Weighted
Percentage
Sample
Size
Weighted
Percentage
Sample
Size
Weighted
Percentage
Total Sample 223,868 100.00 215,860 100.00 203,544 100.00
     Ineligible cases 36,026 15.78 33,284 15.09 32,025 15.40
     Eligible cases 187,842 84.22 182,576 84.91 171,519 84.60
Ineligibles 36,026 100.00 33,284 100.00 32,025 100.00
     Vacant 18,034 49.71 16,796 50.76 16,489 51.71
     Not a primary residence 4,516 12.90 4,506 13.26 4,706 14.69
     Not a dwelling unit 4,626 12.70 3,173 9.33 2,913 8.66
     All military personnel 482 1.22 414 1.21 327 0.93
     Other, ineligible 8,368 23.46 8,395 25.43 7,590 24.00
Eligible Cases 187,842 100.00 182,576 100.00 171,519 100.00
     Screening complete 169,166 89.63 169,769 92.84 157,471 91.86
          No one selected 101,537 54.19 99,999 55.36 90,530 52.11
          One selected 44,436 23.63 46,981 25.46 43,601 25.94
          Two selected 23,193 11.82 22,789 12.03 23,340 13.82
     Screening not complete 18,676 10.37 12,807 7.16 14,048 8.14
          No one home 4,291 2.38 3,238 1.82 3,383 1.90
          Respondent
          unavailable
651 0.36 415 0.24 392 0.24
          Physically or
          mentally incompetent
419 0.24 310 0.16 357 0.20
          Language barrier—
          Hispanic
102 0.06 83 0.05 130 0.09
          Language barrier—
          other
486 0.28 434 0.27 590 0.39
          Refusal 11,097 5.92 7,535 4.14 8,525 4.93
          Other, access denied 1,536 1.08 748 0.45 613 0.35
          Other, eligible 38 0.02 7 0.00 9 0.00
          Other, problem case 56 0.03 37 0.02 49 0.03
Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999, 2000, and 2001.

Table G.2 Weighted Percentages and Sample Sizes for 1999 to 2001 NHSDAs, by Final Interview Code, among Persons Aged 12 or Older
Final Interview Code 1999 NHSDA 2000 NHSDA 2001 NHSDA
Sample
Size
Weighted
Percentage
Sample
Size
Weighted
Percentage
Sample
Size
Weighted
Percentage
Total Selected Persons 89,883 100.00 91,961 100.00 89,745 100.00
Interview complete 66,706 68.55 71,764 73.93 68,929 73.31
No one at dwelling unit 1,795 2.13 1,776 2.02 1,728 2.00
Respondent unavailable 3,897 4.53 3,058 3.52 2,953 3.30
Breakoff 50 0.07 72 0.09 79 0.12
Physically/mentally incompetent 1,017 2.62 1,053 2.57 1,020 2.43
Language barrier—Spanish 168 0.12 109 0.08 190 0.17
Language barrier—Other 480 1.46 441 1.06 470 1.30
Refusal 11,276 17.98 10,109 14.99 10,961 15.60
Parental refusal 2,888 1.01 2,655 0.88 2,517 0.92
Other 1,606 1.53 924 0.86 898 0.86
Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999, 2000, and 2001.

Table G.3 Weighted Percentages and Sample Sizes for 1999 to 2001 NHSDAs, by Final Interview Code, among Youths Aged 12 to 17
Final Interview Code 1999 NHSDA 2000 NHSDA 2001 NHSDA
Sample
Size
Weighted
Percentage
Sample
Size
Weighted
Percentage
Sample
Size
Weighted
Percentage
Total Selected Persons 32,011 100.00 31,242 100.00 28,188 100.00
Interview complete 25,384 78.07 25,756 82.58 23,178 82.18
No one at dwelling unit 322 1.09 278 0.86 254 0.92
Respondent unavailable 872 3.04 617 2.05 551 2.13
Breakoff 13 0.03 18 0.05 17 0.05
Physically/mentally incompetent 244 0.76 234 0.76 219 0.79
Language barrier—Spanish 15 0.03 10 0.03 18 0.08
Language barrier—Other 58 0.18 50 0.20 34 0.11
Refusal 1,808 5.97 1,455 4.52 1,247 4.14
Parental refusal 2,885 9.50 2,641 8.35 2,517 8.95
Other 410 1.33 183 0.59 153 0.64
Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999, 2000, and 2001.

Table G.4 Weighted Percentages and Sample Sizes for 1999 to 2001 NHSDAs, by Final Interview Code, among Persons Aged 18 or Older
Final Interview Code 1999 NHSDA 2000 NHSDA 2001 NHSDA
Sample
Size
Weighted
Percentage
Sample
Size
Weighted
Percentage
Sample
Size
Weighted
Percentage
Total Selected Persons 57,872 100.00 60,719 100.00 61,557 100.00
Interview complete 41,322 67.41 46,008 72.92 45,751 72.29
No one at dwelling unit 1,473 2.25 1,498 2.16 1,474 2.12
Respondent unavailable 3,025 4.71 2,441 3.69 2,402 3.43
Breakoff 37 0.07 54 0.09 62 0.13
Physically/mentally incompetent 773 2.85 819 2.78 801 2.62
Language barrier—Spanish 153 0.13 99 0.09 172 0.18
Language barrier—Other 422 1.62 391 1.16 436 1.43
Refusal 9,468 19.41 8,654 16.22 9,714 16.92
Parental refusal 3 0.00 14 0.01 0 0.00
Other 1,196 1.55 741 0.89 745 0.88
Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999, 2000, and 2001.

Table G.5 Response Rates and Sample Sizes for the 1999 to 2001 NHSDAs, by Demographic Characteristics
  1999 NHSDA 2000 NHSDA 2001 NHSDA
Selected
Persons
Completed
Interviews
Weighted
Response
Rate
Selected
Persons
Completed
Interviews
Weighted
Response
Rate
Selected
Persons
Completed
Interviews
Weighted
Response
Rate
Total 89,883 66,706 68.55% 91,961 71,764 73.93% 89,745 68,929 73.31%
Age in Years                  
     12–17 32,011 25,384 78.07% 31,242 25,756 82.58% 28,188 23,178 82.18%
     18–25 30,439 22,151 71.21% 29,424 22,849 77.34% 30,304 22,931 75.51%
     26 or older 27,433 19,171 66.76% 31,295 23,159 72.17% 31,253 22,820 71.75%
Gender                  
     Male 43,883 31,987 67.12% 44,899 34,375 72.68% 43,949 33,109 71.92%
     Female 46,000 34,719 69.81% 47,062 37,389 75.09% 45,796 35,820 74.58%
Race/Ethnicity                  
     Hispanic 11,203 8,755 74.59% 11,454 9,396 77.95% 10,885 8,777 78.78%
     White 63,211 46,272 67.98% 64,517 49,631 73.39% 63,228 48,016 72.65%
     Black 10,552 8,044 70.39% 10,740 8,638 76.19% 10,584 8,295 74.98%
     All other races 4,917 3,635 59.28% 5,250 4,099 67.31% 5,048 3,841 66.65%
Region                  
     Northeast 16,794 11,830 64.03% 18,959 14,394 71.68% 19,180 14,444 71.02%
     Midwest 24,885 18,103 69.63% 25,428 19,355 73.23% 25,560 19,212 73.25%
     South 27,390 21,018 70.93% 27,217 22,041 76.38% 26,278 20,609 74.44%
     West 20,814 15,755 67.47% 20,357 15,974 72.68% 18,727 14,664 73.51%
County Type                  
     Large metropolitan 36,101 25,901 65.15% 37,754 28,744 71.77% 35,395 26,403 71.00%
     Small metropolitan 30,642 22,612 69.98% 31,400 24,579 74.96% 31,740 24,575 74.66%
     Nonmetropolitan 23,140 18,193 74.97% 22,807 18,441 77.58% 22,610 17,951 76.72%
Source: SAMHSA, Office of Applied Studies, National Household Survey on Drug Abuse, 1999, 2000, and 2001.

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