Answer to Question 1
In order to establish causality, three conditions must be present. First, if A is the causal variable under question, then A the cause and B the effect variables must be correlated. Second, A must precede B in time. Third, other potential causal variables, C, must be ruled out.
Answer to Question 2
Different types of research designs each can contribute different things to our understanding of causal relationships. A type of study called a cross-sectional design observes several cohorts (groups with similar defining characteristics) by taking a snapshot of them at a given point in time whereas longitudinal designs track cohorts over time taking multiple snapshots.
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