Role of Perceived Access and Harm in Marijuana Use Among Adolescents

Thesis (PhD) - Indiana University, School of Health, Physical Education and Recreation, 2006A three-pronged substance abuse prevention intervention was undertaken by a Midwestern, suburban community. Analysis of the long-term impact of the intervention revealed a reduction in alcohol use among students. However, student marijuana use was not impacted as positively. This interdependence between the use of alcohol and marijuana has been reported in the literature, but the role of cognitive factors has not been adequately studied. The current study examined the role of perceived access to alcohol and marijuana and perceived harm associated with alcohol and marijuana use on marijuana use. Data were collected from 11,542 8th - 12th grade students in 1998, 2000, and 2003. Data were examined via two sets of logistic regression analyses investigating (1) the role of alcohol use and access to alcohol and marijuana in past month and lifetime marijuana use and (2) the role of alcohol use and perceptions of harm associated with alcohol and marijuana use in past month and lifetime marijuana use. Results revealed that (a) participants indicating no alcohol use were more likely to report past month and lifetime marijuana use, (b) among older students access to marijuana played a role in past month and lifetime marijuana use only when access to alcohol was limited, (c) perceived harm associated with marijuana played a role in lifetime marijuana use only when perceptions of harm associated with alcohol were low

The Effect of Inappropriate Calibration: Three Case Studies in Molecular Ecology

Time-scales estimated from sequence data play an important role in molecular ecology. They can be used to draw correlations between evolutionary and palaeoclimatic events, to measure the tempo of speciation, and to study the demographic history of an endangered species. In all of these studies, it is paramount to have accurate estimates of time-scales and substitution rates. Molecular ecological studies typically focus on intraspecific data that have evolved on genealogical scales, but often these studies inappropriately employ deep fossil calibrations or canonical substitution rates (e.g., 1% per million years for birds and mammals) for calibrating estimates of divergence times. These approaches can yield misleading estimates of molecular time-scales, with significant impacts on subsequent evolutionary and ecological inferences. We illustrate this calibration problem using three case studies: avian speciation in the late Pleistocene, the demographic history of bowhead whales, and the Pleistocene biogeography of brown bears. For each data set, we compare the date estimates that are obtained using internal and external calibration points. In all three cases, the conclusions are significantly altered by the application of revised, internally-calibrated substitution rates. Collectively, the results emphasise the importance of judicious selection of calibrations for analyses of recent evolutionary events