TREAT = Experiment
A study treats (assigns treatment) → it's an Experiment
ASK = Survey
A study asks questions without changing anything → Sample Survey
WATCH = Observational
A study only observes what already exists → Observational Study
EQUAL = Simple Random
Every member has an equal chance → Simple Random
GROUP+PICK = Stratified
Split into groups, then randomly pick from each → Stratified
GROUP+ALL = Cluster
Split into groups, pick entire clusters → Cluster
EASY = Convenience
Whoever is easiest to reach → Convenience (most biased!)
RULE = Systematic
Start random + follow a fixed rule (e.g., every 5th) → Systematic
VOLUNTEER = Self-Selected
People choose themselves → Self-Selected (biased!)
BIAS = MISrepresent
Bias = results that misrepresent the true population