New programs are settings in which new services are being offered in addition to other existing services. In this setting, the new program may not have been fully thought out at the onset and unforeseen factors may influence its implementation. The evaluator often has little control over the program design or comes to the project too late to design an adequate study. Modification of ongoing programs are situations in which evaluators often have the least influence over program design and implementation. Additionally, vested interests are most prevalent in this situation, making effective evaluation challenging.
Natural experiments entail retrospective study of major changes in health programs.
In this case, the program changes occurred without regard to evaluation and the drawing of conclusions. In these circumstances, adequate data is seldom available. Shortell, Stephen M. and William C. Richardson, Health Program Evaluation. C.V. Mosby Co., 1978.