During the first treatment condition of a within-subjects experiment, the participants learn a new skill that helps improve their performance in later treatment conditions. For this study, the internal validity is threatened by .

Order effects.

Explanation:

Firstly, internal validity will be explained to be the approximate about certain inferences regarding cause relationship and also cause-effects.

In the other hand, order effect which mainly known to be testing effects is seen to only occur in experimental research and also in quasi-experimental research designs that have more than one stage. Furthermore, research designs of this form is seen to involve a pre-test and also a post-test. Circumstances of this kind pulls the fact that the person taking part in the research is tested more than once can influence their scores in the post-test, therefore confounding the results.

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