The 50 subjects included in the experimental group scored high in the pre-test,  however, dropped out before the completion of the post-test.

Study on the effect of foods with chorella to height of the preschooler. The subjects may increase in height even if the foods they take does not have chorella as they age.

Researcher offer a seminar to help the adult stop smoking. The 10 subjects for experimental group volunteer that they are willing to stop smoking. The other 10 subjects for the control group have not shown desire to stop smoking.

Employees may work harder and more diligently knowing their manager is closely watching, or children behave better because their parents are looking on.

Examples of these characteristics include anxiety, a need for approval, hostility, warmth, or authoritarianism, all of which may affect the behavior or responses of the subjects in an experiment.

For example, as people become elderly, there can be a more rapid deterioration in certain physical characteristics such as vision, hearing, taste, and even memory. This may negatively impact their performance during an experiment.

Participants showed higher productivity at the end of the study because the same test was administered. Due to familiarity, or awareness of the study’s purpose, many participants achieved high results. ( The pre-test influences the outcomes of the post-test.)

Different measures are used in pre-test and post-test phases

in a Web-based instruction project entitled Eruditio, it started with 161 subjects and only 95 of them completed the entire module. Those who stayed in the project all the way to end may be more motivated to learn and thus achieved higher performance.