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So far we have been concern with cross sectional surveys, the most common kind. Sometimes we wish to study if there is a trend over time, so we need a follow up survey or a panel survey. In a follow up survey, the same subjects are followed on time, it is a cohort study. In a panel survey, only a subset is studied the next time and a new group is sampled to complete the number. In a cohort each subject must be interviewed 3 or 4 times which causes depletion or losses to follow up. In a panel each subject is studied only twice and we must reconstruct the trend from picewise information.