In posttest only quasi-experimental designs,the major threat to internal validity is that it is virtually impossible to conclude that changes are the result of the manipulation of the independent variable alone.

Question

Quizplus · Accepted Answer

The posttest only design does not include a pretest measure, which could help establish baseline levels of the dependent variable and rule out alternative explanations for any changes observed in the posttest. Without a pretest, researchers cannot be sure if the groups were similar prior to the manipulation of the independent variable or if other factors influenced the outcome. This lack of control over extraneous variables is the major threat to internal validity in this design.

In Posttest Only Quasi-Experimental Designs,the Major Threat to Internal Validity