Mediating mechanisms in a school-based drug prevention program: First-year effects of the Midwestern Prevention Project.

Abstract

Describes (a) the effects of a social-influences-based drug prevention program (the Midwestern Prevention Project) on the mediating variables it was designed to change and (b) the process by which the effects on mediating variables changed use of drugs (tobacco, alcohol, and marijuana). Students in 42 middle schools and junior high schools in Kansas City, Missouri, and Kansas City, Kansas, were measured in the fall of 1984 (N = 5,065) and again 1 year later (N = 5,008) after 24 of the schools had been through the program. Compared to students in control schools, students in program schools became less likely to express belief in the positive consequences of drug use, less likely to indicate that they would use such drugs in the future, more likely to report that their friends were less tolerant of drug use, and more likely to believe that they were better able to communicate with their friends about drug or school problems. Change in perceptions of friends' tolerance of drug use was the most substantial mediator of program effects on drug use. There was evidence that intentions to use and beliefs about the positive consequences of use may also mediate program effects on drug use.

Confidence Limits for the Indirect Effect: Distribution of the Product and Resampling Methods

“…For nonzero values of both ␣ and ␤, simulation studies suggest that the variance estimator has relative bias less than 5% for sample sizes of 100 or more in a single indirect effect model (MacKinnon et al, 1995) and for sample sizes of 200 or more in a recursive model with seven total indirect effects (Stone & Sobel, 1990). In many studies, the indirect effect is divided by its standard error and the resulting ratio is then compared to the standard normal distribution to test its significance, z = / => => I (Bollen & Stine, 1990;MacKinnon et al, 1991;Wolchik, Ruehlman, Braver, & Sandler, 1989). Confidence limits for the indirect effect lead to the same conclusion with regard to the null hypothesis.…”

Distribution of the product confidence limits for the indirect effect: Program PRODCLIN

“…For nonzero values of both and , Monte Carlo studies suggest that all three variance estimators appear to have relative bias of less than 5% for a sample size of 100 or more in a simulation study of the single indirect-effect model (MacKinnon et al, 1995) and a sample size of 200 for the multivariate delta standard error in a simulation study of a recursive model with seven indirect effects (Stone & Sobel, 1990). In many studies, the indirect effect is divided by its standard error and the resulting ratio is then compared with the normal distribution to test its significance (Bollen & Stine, 1990;MacKinnon et al, 1991;Wolchik, Ruehlman, Braver, & Sandler, 1989). Confidence limits for the indirect effect lead to the same conclusion with regard to the null hypothesis.…”

Demonstration and evaluation of a method for assessing mediated moderation

“…Mediational processes guide the development and evaluation of preventive intervention trials (Lipsey, 1993;MacKinnon & Dwyer, 1993;MacKinnon, Taborga, & Morgan-Lopez, 2002;West & Aiken, 1997). In etiological studies, mediation analyses help identify links between risk factors and outcomes (see, e.g., MacKinnon et al, 1991;Wolchik et al, 1993).…”