Cross-sectional:
1. involve observation of all of a population OR a subset
2. takes place at one specific time point
3. aim: provide data on the entire population under study
4. descriptive
5. can be used to describe: RR, OR, absolute risk
6. used to describe some features of the population:
a. prevalence of illness
b. support inferences of cause & effect (answer Q about cause of disease)
7. not suitable to study rare diseases
8. moderately expensive
9. rely on memory introduce bias (recalling the past)