An educational psychologist is investigating the effectiveness of a new gamified mobile app compared to traditional paper flashcards on the vocabulary retention of foreign language learners. The psychologist utilizes an independent groups design, where different participants are randomly assigned to either use the gamified app or the paper flashcards.

The researcher is concerned that participant variables, such as individual differences in baseline memory capacity or pre-existing language aptitude, may have confounded the results.

Outline two different ways in which the psychologist could modify the experimental design of this study to address this issue.