1. Design of Experiments

2. 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

3. 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

4. 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.

5. 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

6. 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.

7. 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

8. 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