The question as to whether the experimental treatment was the "sole cause" of the changes in the dependent variable is the basic issue in internal validity.

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Internal validity is concerned with whether the experimental treatment is responsible for the changes in the dependent variable, and whether any other extraneous factors may have influenced the results. The question of whether the experimental treatment was the "sole cause" is central to establishing internal validity.

The Question as to Whether the Experimental Treatment Was the "Sole