I am looking at the effects of resistance to a drug intended to increase serotonin levels on the course of depression. Basically, every participant (N=20) takes a pre measurement of serotonin levels (concentration) and does a questionnaire to assess for depression levels (scored out of 50). Then, every participant is given the drug intended to increase serotonin levels. We then measure both their serotonin levels and depression score on the questionnaire after 15 weeks of taking the drug. We want to see if the change in depression scores (either worsened or improved depression) depends on their response to the drug (response would be whether their serotonin levels is increased, decreased, or stays the same) after the 15 weeks. Overall, our hypothesis would be that resistance to the drug (meaning it doesn’t effect their serotonin levels or even decreases serotonin levels) results in worsened depression (meaning higher depression scores after the 15 weeks) and that a positive response to the drug (meaning increased serotonin levels) improves depression scores.
The problem is I cannot find any statistical test to answer this research question.