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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 comparison conditions.
Math achievement
Math achievement is measured by three types of student outcome measures:
- Standardized, nationally normed achievement tests that are appropriate for elementary students (TerraNova CTBS, Iowa Test of Basic Skills)
- Standardized state or local tests of math achievement (Massachusetts Comprehensive Assessment System)
- Research-based or locally developed tests or instruments that assess students' mathematical knowledge or skills.
We reviewed evidence of the effects on math achievement outcomes for five programs and, for these five, the average improvement index ranged from -2 to +6 percentile points (figure 1).
