Internal and external validity in two studies that compared treatment methods.

Research comparing the effectiveness of two treatments offers both strengths and weaknesses for occupational therapy. Although it is worthwhile to determine which of two treatments works best for a particular problem, methodological problems may arise that preclude a valid conclusion. To draw valid conclusions from research, criteria for internal validity and external validity must be satisfied. The two preceding articles in this issue are examples of studies that used between-groups experimental methodology to compare the effectiveness of two different treatments. This paper evaluates the above-mentioned studies on the basis of principles of internal and external validity. One of these studies (Jongbloed, Stacey, & Brighton, 1989) was truly experimental, whereas the other study (Groves & Rider, 1989) was quasi-experimental. Results from both studies were similar because, in each study, both of the treatment groups improved, but there were no significant differences between treatments. Absence of a true control group in both studies presented limitations on the conclusion that both treatments worked equally well.