As a matter of semantics, we have not proved the null hypothesis to be true (that substance abuse treatment has no effect). It may turn out to be extremely effective for ciother group of prisoners. Or perhaps many more of the prisoners in this treatment group would have been rearrested if they had not received the treatment. In any case, i the basis of the data collected, we have merely failed to reject our null hypothesis, here is a similar, distinction Assumption that substance abuse treatment is not an effective tool for keeping ex-offenders from returning to prison.