• Anecdotal data come from stories or reports about cases that do not necessarily represent a larger group of cases.
• Available data are data that were produced for some other purpose but that may help answer a question of interest.
• A sample survey collects data from a sample of cases that represent some larger population of cases.
• A census collects data from all cases in the population of interest.
• In an experiment, a treatment (an intervention) is imposed and the responses are recorded.
• Confounding occurs when the effects of two or more variables are related in such a way that we need to take care in assigning the effect on the response variable to one or to the other.