The present study addressed the importance of developing general outcome measures in mathematics using an estimation task. The study examined a general outcome measure estimation probe using data from 111 sixth grade students from a suburban area in Iowa. The estimation probe was compared to a formal test of estimation to establish validity. A moderately strong relation was obtained between the estimation probe and the formal estimation test. Further comparisons were made between the estimation probe and three criterion measures including teacher ratings, grade point in mathematics and composite scores in mathematics from the Iowa Test of Basic Skills. The estimation test in this study was also compared to the same criterion variables. The difference between each of the correlations was determined to be statistically insignificant. These results indicated the probe in this study could be used interchangeably with the formal estimation test as an indicator of student proficiency in mathematics.