Random Assignment
To ensure that there are no unexpected differences between the experimental and control groups, good experimental design uses random assignment to determine which participants are in each group. This helps to minimize the impact of other variables that the researcher has no control over.
Do not confuse this with random sampling! (In fact, most experiments do not use random sampling to find their sample.)
- Random sampling = WHO gets into the study (selection from population)
- Random assignment = WHERE participants go after they have already been selected for the study (placement into groups)
Replication
In addition to random assignment, replication helps ensure that the results of an experiment are truly caused by the change in the factor of interest and not by other hidden factors or natural variation in data. Replication can be accomplished in two ways:
- Having multiple participants experience each treatment condition within one experiment
- Repeating the entire experiment with new participants
Both approaches ensure results are consistent and not due to chance. For example, having 30 people in each group instead of 3 (within-experiment replication), or running the same experiment with multiple groups of participants (repeating the experiment).
essentials of experimental design
The key components of a well-designed experiment are: control, randomization, and replication.