In a nonequivalent control group pretest-posttest design, the greatest threat to internal validity is

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Local history refers to specific events that occur in the environment of the experimental group but not in the control group during the course of the study. These events can affect the outcome, making it difficult to determine if changes in the experimental group are due to the treatment or to these external events. This is a significant threat to internal validity in nonequivalent control group designs because the groups are not randomly assigned, increasing the likelihood that differences between groups are due to factors other than the treatment.

In a Nonequivalent Control Group Pretest-Posttest Design, the Greatest Threat