Design of Experiments

Definitions Experimental Units = the individuals that are studied. These can be people, animals, plants, or things. When the experimental units are people, they are sometimes called subjects Outcome (or Response) = what is measured on each experimental unit Treatments = The procedures applied to each experimental unit. There are always two or more treatments

Definitions Randomized Experiment = a study in which the investigator assigns the treatment to the experimental units at random Observational Study = One in which the assignment to treatment groups is not made by the investigator Placebo = a harmless tablet that looks like the drug but has no medical effect

Summary In a randomized experiment, if there are large differences in outcomes among the treatment groups, we can conclude that the differences are due to the treatments.

Definitions Double-Blind = Neither the investigators nor the subjects know who has been assigned to which treatment Completely Randomized Experiment = No restriction on which subjects may be assigned to which treatment Randomized Block Experiment = Subjects are split into blocks and then randomly chose from within each block

Definitions Confounding = Difficulty in telling whether a difference in the outcome is due to the treatment or to some other difference between the treatment and control groups Confounder = A variables that is related to both the treatment and the outcome.

Definitions Cohort Study = a group of subjects (the cohort) is studied to determine whether various factors of interest are associated with an outcome Types of Cohort Studies Prospective = subjects are followed over time Cross-sectional = measurements are taken at one point in time Retrospective = subjects are sampled after the outcome has occurred

Definition Case-Control Study = Two samples are drawn. One sample consists of people who have the disease of interest (the cases) and the other consists of people who do not have the disease (the controls). Investigators look back in time to see if a factor of interest differs between two groups