Back to Education
Up to Table of Contents
Ahead to Marijuana:

9. TRENDS IN INITIATION OF DRUG USE

Estimates of drug use incidence, or initiation, provide another measure of the Nation's drug problem. They can suggest emerging patterns of use among young people. In the past, increases and decreases in incidence have usually been followed by corresponding changes in the prevalence of use. SAMHSA recently released a detailed report on incidence trends based on 1991-93 data, covering the period 1919 through 1992. Using the 1994 and 1995 NHSDA data, it is now possible to update those earlier estimates using the improved NHSDA questionnaire data, as well as develop estimates for 1993 and 1994.

Details of the methodology are available in Trends in the Incidence of Drug Use in the United States, 1919-1992, released earlier this year. Briefly, the estimates are based on the NHSDA questions on age at first use. Using each respondent's reported age at first use in conjunction with his/her age and interview date, the respondent's year of first use of each drug was determined by subtracting their age from the interview year and then adding the age at first use. By combining all respondents and applying sample weights, estimates of the number of new users of each drug for each year were made. These estimates include new users at any age, including under age 12. In addition, the average age of new users in each year and age-specific rates of first use were estimated. These rates are presented in this report as the number of new users per 1,000 person-years of exposure. The numerator of each rate is the number of persons in the age group who first used the drug in the year (times 1,000), while the denominator is the number of persons who were exposed to the risk of first use during the year, adjusted for their estimated exposure time in years. Persons who first used the drug in a prior year have zero risk of first use in the current year, and persons who still have never used the drug by the end of the current year had 1 year of exposure to risk. Persons who first used during the year are assumed to have a half year of exposure to risk.

The incidence estimates are based on retrospective reports of age at first drug use by survey respondents interviewed during 1994-95, and may therefore be subject to several biases, including bias due to differential mortality of users and nonusers of each drug, bias due to memory errors (recall decay and telescoping), and underreporting bias due to social acceptability and fear of disclosure. See Appendix 2, Section III for a discussion of these biases. As is explained in Appendix 2, it is likely that some of these biases, particularly telescoping and underreporting because of fear of disclosure, may be affecting estimates for the most recent years more significantly. Thus, estimates for these recent years (i.e., 1992-94) may be less stable than other estimates, and should be interpreted with caution.

Back to Education
Up to Table of Contents
Ahead to Marijuana: