Background:
The purpose of this research is to examine the impact of a Standards-based elementary mathematics curriculum on third grade students’ mathematics performance.

Materials and methods:
A total of 707 students participated in this study. Of this total, 368 students were from eight schools located within the same school district in a racially and ethnically diverse large city in the Midwest. Another 339 of the students were from four schools located in two different school districts in a middle- to high-SES, largely white, suburban area in the Northeast.

Results:
Third grade students using Investigations curriculum outperformed in most cases and performed the same in the other cases matched comparison groups who were using a range of conventional curricula on the mathematics assessment. The Investigations groups did the same or better on the decontextualized, contextualized, and algebraic-reasoning constellations compared with their counterparts.

Conclusions:
In conlusion, the revised Investigations curriculum is not as effective with the low SES African-American students as it is for middle to high SES, white students.The curriculum fidelity measures did not differentiate achievement differences.