Is it necessary to ensure generalization of study findings beyond the sample to a larger population for a program evaluation study?

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No, it is not necessary. However, we have to distinguish two issues with generalizability in program evaluation contexts.
1.Whether the findings of the evaluation can be generalized to the same (or similar) larger population that will be the potential clients of the program in the future
2.Whether the findings of the evaluation can be generalized to the clients who received the services but did not participate in the evaluation