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.