Completely Randomized Design (CRD)

All experimental units are randomly assigned to treatments with no grouping.

Randomized Block Design

1. Group subjects into blocks of similar individuals
2. Randomly assign treatments within each block
3. This reduces variability due to the blocking variable

Example: Block by gender, then randomly assign treatments within each gender group.

Note: Blocking is like stratifying in sampling — but blocks are for experiments, strata are for surveys.

Matched Pairs Design

A special case of blocking where:

• Each "block" has only 2 units (matched on key characteristics)
• One gets Treatment A, the other gets Treatment B

OR:

• Each subject serves as their own control (before/after)

Inference and Scope

AP Tip: A well-designed experiment has random assignment (to establish causation) and ideally random selection (to generalize to a population).