What Works Clearinghouse Standards

'Meets WWC Standards Without Reservations' is reserved for well-conducted randomized controlled trials with low attrition and no disqualifying confounds — the strongest causal evidence. 'Meets WWC Standards With Reservations' applies to strong quasi-experimental designs (or RCTs compromised by attrition) that demonstrate baseline equivalence of the analysis sample. 'Does Not Meet WWC Standards' indicates the study cannot support a causal conclusion under the criteria, so its findings are not used for that purpose in WWC reviews.

Attrition — participants leaving the study — can undermine even a randomized trial, because those who drop out may differ systematically between groups, reintroducing the selection bias randomization was meant to remove. The WWC applies an empirically derived attrition boundary on overall and differential attrition; trials within the acceptable range can still meet standards (sometimes with reservations and a baseline-equivalence check), while excessive attrition can disqualify an otherwise strong design. This focus reflects that high attrition is a leading threat to causal validity.

Not necessarily. The standards assess internal validity — whether the study credibly estimates a causal effect in its own sample and setting. They say little about external validity, that is, whether the effect would replicate with different students, teachers, or conditions. A program with strong WWC evidence is a better bet than one without, but local context, implementation fidelity, and population differences still matter. The rating is about the trustworthiness of the finding, not a guarantee of transfer.