Stanley conducts a study in which he assesses teasing in a sample of first-graders during the first week of school and again three months later. Because the study is longitudinal, Stanley has to worry about which of the following threats to internal validity?

Maturation

Explanation:

In terms of internal validity, maturation is type of treat that impacts research because certain physical changes or mental modification tend to occur in participants which tends to alter the results.

According to the question, Stanley should worry about maturation trait as he is conducting experiment on first grade student .He conducts research of teasing on them in first week and then three months later which could have brought some mental changes or physical changes in student .Thus results can get modified.  

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