Final answer:
In between-subjects experiments, groups must be randomly assigned, have an equivalent control group, and differ only by the experimental manipulation to ensure reliable results.
Step-by-step explanation:
In between-subjects experiments, the researcher has both the opportunity and the responsibility to create groups that are equivalent. The separate groups must be comparable in all respects except for the treatment they receive. This means groups should be: 1) randomly assigned, ensuring all potential lurking variables are evenly distributed, 2) have a control group that is identical to the experimental group except it does not receive the independent variable, and 3) the only difference between the groups should be the experimental manipulation, so any outcomes are attributable to the treatment and not to chance or other factors. Utilizing these three criteria, the study can establish a cause-and-effect relationship between the variables being examined.