1. Study design.

Study design	Description
Cohort study	An observational study in which a defined group with ≥ 1 samples of people (the cohort) is followed over time. The outcomes of people in subsets of this cohort might be compared, to examine people who were exposed or not exposed (or exposed at different levels) to a particular intervention or other factor of interest. A prospective cohort study assembles participants and follows them into the future. A retrospective (or historical) cohort study identifies participants from past records and follows them from the time of those records to the present. Because participants are not allocated by the investigator to different interventions or other exposures, adjusted analysis is usually required to minimise the influence of other factors (confounders).
Patient‐control study	A study that compares people with a specific disease or outcome of interest (cases) to people from the same population without that disease or outcome (controls), and which seeks to find associations between the outcome and prior exposure to particular risk factors. This design is particularly useful when the outcome is rare and when past exposure can be reliably measured. Patient‐control studies are usually but not always retrospective.
Cross‐sectional study	Studies in which the presence or absence of disease or other health‐related variables are determined for each member of the study population or in a representative sample at one particular time. This contrasts with cohort studies, which are followed over a period of time.

Taken from the Cochrane Glossary.