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The most effective way to address the issue of internal validity in program evaluation design is through the use of a comparison group. Internal validity refers to the extent to which a study demonstrates that the observed effects or outcomes are directly attributable to the intervention rather than other confounding factors.

By including a comparison group, evaluators can more accurately isolate the impact of the program. The comparison group allows researchers to observe what happens in a similar group that does not receive the intervention, helping to control for external variables that might influence outcomes. This direct comparison helps ascertain whether any changes can be attributed to the intervention itself.

The other factors contribute to the overall evaluation process but do not directly address internal validity as effectively. Reliable and valid measurement instruments are crucial for accurately capturing data, but they do not control for other variables that might affect results. Well-written process objectives focus more on implementation rather than the causal relationships necessary for evaluating outcomes. Ensuring generalizability pertains to external validity, which is about whether the findings can be applied to larger populations but does not inherently address internal validity. Therefore, the use of a comparison group stands out as the most direct method for reinforcing the internal validity of a program evaluation.