This is especially important in this example because the life of any family is complex and involves many events and influences that might lead to a change in functioning, regardless of program participation. The randomized experiment is the most powerful research design for determining whether a program is effective. Random assignment is not always possible or practical, however. There is always the danger that the randomization procedure will not work, and that the groups are still actually different. This means that the program's impact is still unknown because scores might differ due to some other differences between the groups besides those that might be attributed to the program.
True experiments are likely to be quite impractical for evaluating secondary prevention programs.
They can be quite expensive and difficult to implement, should not be attempted without the assistance of an outside consultant. Further, there are often easier and less expensive ways to gather the same information. However, a small-scale true experiment on a primary prevention program might be feasible.