Gain score

From EvaluationWiki

The difference between the score on a pre-test and a post-test, used to determine the extent of the change due to the intervention or process that occurred between the two tests. Of course, if the scores are raw scores rather than true scores, some of the change will be due to statistical artifacts such as regression to the mean (Stat.). And even if the difference is a true score difference, it may be due to maturation, peer tutoring, or some other intervention that happens to have occurred during the time period between tests. So gain scores can be misleading.