Cross sectional study: An observational study in which each respondent is ‘interviewed’ only once. Measure: Prevalence

Case-Control Study: An analytical study in which those with disease/ condition (cases) are historically compared with those without disease/ condition (controls) for exposure to a putative/suspected factor. Measure of Risk/ Association: Odds Ratio

Cohort study: An observational study in which a disease free group (‘cohort’) is observed over a long time (often years-decades). Each respondent provides multiple observations. Measures of Risk/ Association: Risk Ratio (Relative Risk), Attributable Risk

Interventional Trial: An experimental study in which participants receive placebo/ intervention, or both to test a (causal) hypothesis. Multiple observations are made over the duration of the study.

Randomized Controlled Trial: An interventional study in which subjects are randomized to receive/ not receive 1 or more interventions/ placebo under controlled conditions.

Single Masked/Blind Randomized Controlled Trial: A Randomized Controlled Trial in which (typically) the subject does not which arm of the study (s)he has been assigned to (and therefore, what (s)he is receiving). Minimizes subject Bias.

Double Masked/ Blind Randomized Controlled Trial: A Randomized Controlled Trial in which (typically) the subject and the observer don’t know which arm of the study the subject is in (and therefore what (s)he is receiving). Minimizes Subject Bias and Observer Bias.

Triple Masked/ Blind Randomized Controlled Trial: A Randomized Controlled Trial in which the subject, the observer, and the analyst, all don’t know who is receiving what intervention.

Ecological Study: A study that compares population groups and identifies differences between them, often suggesting a relationship between a factor and a disease/ condition. Findings are often not valid at the individual level- ‘Ecological fallacy’.

Clinical Trial: An interventional study conducted in a clinical setting.