In contrast to cross-sectional studies, longitudinal studies are designed to observe a sample over an extended period or to study changes over time. In general, longitudinal studies are often the best way to study changes over time . To examine the effect of alcohol advertising and marketing on alcohol consumption in young people longitudinal studies are especially valuable because they capture what happens to individuals over time and can demonstrate whether individuals who differ in their exposure to advertising differ in their future drinking behaviour . Longitudinal studies provide the highest level of evidence that is available for evaluation of advertising and marketing exposure and subsequent drinking behaviour. If such studies are well designed, conducted and analysed they can provide supportive evidence for a causal relationship between a particular exposure and an outcome . Well conducted and designed longitudinal studies have a representative sample with a minimum of attrition. Besides a reliable measurement of the main predictors and outcomes, they include main confounding variables as well.
Longitudinal studies can be divided in three types of studies: trend studies, cohort studies and panel studies . Trend studies are longitudinal studies that examine population changes over time. In contrast to trend studies, cohort studies are used to examine a specific subpopulation over time. The latter type of study is more suited to examine the effect of alcohol advertising on alcohol consumption in young people, since it makes it possible to examine the effect on specific age group. Prospective cohort studies define the groups of interest before the study is conducted. Although quite similar to trend and cohort studies, panel studies examine the same set of respondents at each measurement moment, which is not the case in trend and cohort studies. Most longitudinal studies on the effects of alcohol marketing, although frequently mentioned as cohort studies, are panel studies which follow the same set of respondents during several months or years.
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