In a within-subjects experiment comparing two treatments, the researcher is concerned that the results may be confounded by outside events that occur between the first and second treatments. one way to reduce the likelihood that internal validity will be influenced by an outside event is to

between-subjects design

Explanation:

According to my research on different experimental designs, I can say that based on the information provided within the question the best way to make sure that the internal validity will not be influenced by an outside event is to switch to a between-subjects design. This type of design is an experiment design that has two or more groups of subjects each being tested by a different testing factor simultaneously. Since they are done simultaneously it significantly reduces outside influence.

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