1. the error that occurs when there is a discrepancy between the research focus and the hypothesis actually tested. For example, a Type III error would have happened if a researcher collected data on individual differences within a sample and determined the causes of variation but the question of interest concerned differences between populations. In other words, a Type III error involves providing the right answer for the wrong question. Also called Type 0 error.

2. the error that occurs when a researcher correctly rejects the null hypothesis of no difference between samples but then makes an incorrect inference about the direction of the difference. Researchers investigating the direction of a relationship (e.g., “Which is more?” or “Which is better?”) will make a Type III error if they use a nondirectional test to make a directional decision.

3. in clinical tests, attributing a lack of results to the weakness of a treatment when, in fact, the problem was that the treatment was not administered properly.