A clinical researcher wants to determine if antidepressants should be used in conjunction with therapy to address post-traumatic stress disorder (PTSD) among returning veterans. She designs an experiment with 16 volunteers who had been discharged from the military and diagnosed as suffering from PTSD. After a period of 1 year either in therapy or not and either on antidepressants or not, the researcher measures their PTSD symptoms on a scale ranging from 1 (low) to 15 (high). Using a two-way ANOVA, test the null hypothesis that therapy and antidepressants have no effect, either alone or in combination, on PTSD symptoms.

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No Antidepressants No Therapy 9 10 8 13 Therapy 9 10 14 9 Antidepressants No Therapy Therapy 7 8178 10 11 6 8 11