Entire books have been written on experimental design, and some of them are quite hefty! Obviously, I cannot do justice to this very important topic on one webpage. Important issues that we will have to discuss as we encounter them are the following:

• Were subjects randomly sampled from some population or not?
• Were subjects randomly assigned to groups? If not, why not? Was it because of sloppy experimental procedure, or were the subjects self-selected (intact groups)?
• How many groups were subjects assigned to? (The important distinction here is between two groups and more than two groups.)
• How many independent variables are there? (The important distinction here is between one IV and more than one IV. If there is more than one IV, was the experimental design factorial?)
• Were subjects measured under only one condition of the experiment (between groups or between subjects design), or were subjects measured in more than one condition of the experiment (within subjects or repeated measures design)?

These issues will determine what type of statistical analysis is appropriate and how the results of that analysis are interpreted.