Results First Clearinghouse Database Tiers
Highest rated (green): The clearinghouse assigned the intervention its highest rating. In general, this requires one to two evaluations that: a)use the strongest research designs, including randomized control trials or high-quality quasi-experimental designs; and b)show that the intervention had a statistically significant positive impact.
Second-highest rated (yellow): The clearinghouse assigned the intervention its second-highest rating. This generally requires an evaluation that used a quasi-experimental design and showed that the intervention had a positive impact.
No evidence of effects (grey): The clearinghouse found the intervention to have no statistically significant effects based on at least one evaluation that used a randomized control trial or a quasi-experimental design.
Mixed effects (blue): This category is only used for interventions reviewed by What Works Clearinghouse. It is applied when two or more evaluations using randomized control trials or quasi-experimental designs found inconsistent effects of a given intervention. That is, one study showed a positive effect and ether another showed a negative effect or at least two others showed indeterminate effects.
Negative effects (red): The clearinghouse determined that the intervention had negative effects, which could be statistically significant or not, based on at least one evaluation that used a randomized control trial or a quasi-experimental design.