Randomized trials of social programs yield internally valid estimates of causal impacts on key outcomes. While estimates of gross impact serve as useful summaries, program developers require deeper information to drive improvement efforts, especially when no impact is observed. The first main goal of this work is to present a seven-stage diagnostic method for assessing process bottlenecks in experiments. Designed for programs still in development, the troubleshooting sequence uses mixed methods to assess where in a program's logic model the process is compromised. It includes post-experimental methods that are built into the design, to account for impact variation and test where effects are intensifying or diminishing. The second main purpose of this work is to demonstrate one such method in detail. The approach tests the relationship between fidelity of program implementation and impact. First, levels of achieved fidelity in the treatment group are modeled in terms of informative baseline covariates. The model is then used to index fidelity in both conditions. Informed only by pre-randomization characteristics of individuals, the model-based fidelity scores are unbiased by endogeneity, and allow assessment of whether impacts on key outcomes vary by levels of fidelity. Results can help program developers focus improvement efforts. We illustrate the seven-step diagnostic process through a randomized trial of the Internet-Based Reading Apprenticeship Improving Science Education (iRAISE) program. Eighty-two high school science teachers and 1468 students were randomly assigned to a literacy program or control. There was no overall impact on achievement. Applying the diagnostic process revealed this was not due to a weak program contrast between conditions, or an inadequate assessment; rather, lower-than-expected impact was likely due to weaker than intended implementation.

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