The second criterion of an effective intervention involves knowledge of outcomes. Because interventions are intended to produce specific results, they must be based on valid knowledge that those outcomes actually can be produced. Otherwise there is no scientific basis for designing an effective OD intervention. Unfortunately, and in contrast to other applied disciplines such as medicine and engineering, knowledge of intervention effects is in a rudimentary stage of development in OD. Much of the evaluation research sufficient rigor to make strong causal inferences about the success or failure of change programs. (Chapter 7 discussed how to evaluate OD programs.) Moreover, few attempts have been made to examine the comparative effects of different OD techniques. All
of these factors make it difficult to know whether one method is more effective than another.