confounded effects
the indivisible effects of two or more predictor or independent variables on a single response or dependent variable. That is, the unique influence of one predictor cannot be separated out from that of the others. For example, assume a researcher is examining the effectiveness of a hunger-reduction pill for weight loss. If some of the participants receiving the pill discover they are part of the treatment group and concurrently adopt better eating habits and increase their exercise levels, these additional healthy activities will also affect weight loss. Since those in the control group (receiving a placebo sugar pill) are not engaging in these same supplemental activities, effects of eating habits, exercise levels, and the actual treatment will have become confounded.