Average improvement indices

The WWC computes an average improvement index for each study, as well as an average improvement index across studies of the same intervention (see the Technical Details of WWC-Conducted Computations).

The improvement index represents the difference between the percentile rank of the average student in the intervention condition and the percentile rank of the average student in the comparison condition. It can take on values between -50 and +50, with positive numbers denoting results favorable to the intervention group. Unlike the rating of effectiveness, which is based on four factors, the improvement index is based only on the size of the difference between the intervention and the comparison conditions.

Math achievement

Math achievement includes three types of outcome measures:

• Standardized, nationally normed achievement tests that are appropriate for elementary students (e.g., Comprehensive Test of Basic Skills, Wide Range Achievement Test)
• Standardized state or local tests of math achievement
• Research-based or locally developed tests or instruments that assess students' mathematical concepts or skills

We reviewed math achievement outcomes for 7 curricula, and the average improvement index ranged from -2 to +14 percentile points (figure 1). ³