I. Between-subject design
1. With Subject Independent Variables -- between-subject designs are sometimes used because they are the only option. If the IV is a subject (or a non-manipulated) variable, we cannot assign people to both levels.
2. With certain Manipulated IVs -- It is sometimes the case that when subjects participate in one condition, the experience they gain will make it impossible for them to participate in other conditions. They have changes somehow and there is no going back.
- Requires a lot of subjects
- The differences in results may be attributed to differences between the groups and not to the IV – here we need to create EQUIVALENT GROUPS
4. The problem of creating equivalent groups
- Random assignment
- Matching -- subjects are paired together on some trait such as anxiety level. Then one member of each pair is randomly assigned to one of the groups.
II. Within-Subject Designs
- Number of participants
- No equivalent groups problem