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III. Nonsampling Error

Nonsampling errors occur from nonresponse, coding errors, computer processing errors, errors in the sampling frame, reporting errors, and other errors. Nonsampling errors are reduced through data editing, statistical adjustments for nonresponse, and close monitoring and periodic retraining of interviewers.

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.

Of the 56,469 eligible households sampled, 53,201 were successfully screened for a screening response rate of 94.2%. In these screened households, a total of 22,016 sample persons were selected, and completed interviews were obtained from 17,747 of these sample persons, for an interview response rate of 80.6%. 2,004 (9.1%) of sample persons were classified as refusals, 1,394 (6.3%) were not available or never at home, and 871 (4.0%) did not participate for various other reasons, such as physical or mental incompetence or language barrier. Response rates were highest in younger age groups. Response rates were also higher among Hispanics (83%) than among blacks (80%) and whites (80%).

Among survey participants, item response rates were above 98% for most questionnaire items. However, inconsistent responses for some items, including the drug use items, are common. Estimates of drug 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 judgement on the part of survey analysts and is a potential source of nonsampling error. A typical occurrence is when a respondent reports their most recent use of a drug as more than a month ago, but in a later question they report having used in the past month. (This could occur because the interviewer may have developed greater rapport with the respondent in the latter stages of the interview, leading to more openness on the part of the respondent.) This respondent would be considered a past month user. For 1995, 21% of the estimate of past month marijuana use and 37% of the past month cocaine use estimate is based on such cases.

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. No adjustment to NHSDA data is made to correct for this (Appendix 4 lists a number of references addressing the validity of self-reported drug use data). The methodology used in the NHSDA has been shown to produce more valid results than other self-report methods (e.g., by telephone) (Turner, Lessler, and Gfroerer 1992; Aquilino 1993). 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).

The incidence estimates discussed in section 9 of this report are based on retrospective reports of age at first drug use by survey respondents interviewed during 1994-95, and may be particularly subject to several biases.

Bias due to differential mortality occurs because some persons who were alive and exposed to the risk of first drug use in the historical periods shown in the tables died before the 1994 and 1995 NHSDAs were conducted. This bias is probably very small for estimates shown in this report. Incidence estimates are also affected by memory errors, including recall decay (tendency to forget events occurring long ago) and forward telescoping (tendency to report that an event occurred more recently than it actually did). These memory errors would both tend to result in estimates for earlier years (i.e., 1960s and 1970s) that are downwardly biased (because of recall decay) and estimates for later years that are upwardly biased (because of telescoping). There is also likely to be some underreporting bias due to social acceptability of drug use behaviors and respondents' fear of disclosure. This is likely to have the greatest impact on recent estimates, which reflect more recent use and reporting by younger respondents. Finally, for drug use that is frequently initiated at age 10 or younger, estimates based on retrospective reports one year later underestimate total incidence because 11 year old children are not sampled by the NHSDA. Prior analyses showed that alcohol and cigarette (any use) incidence estimates could be significantly affected by this. Therefore, for these drugs no 1994 estimates were made, and 1993 estimates were based only on the 1995 NHSDA.

Overall, these biases are likely to have the greatest effect on the most recent estimates, i.e., 1992-94, primarily because they reflect recent drug use and because they are heavily based on the reports of adolescents. Thus, the estimates for recent years may be less reliable than estimates for earlier periods.

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