This paper investigates the power of the computer guided clinical interview (CI) and new curriculum based measurement (CBM) measures to identify and help children at risk of low mathematics achievement. We use data from large numbers of children in Kindergarten through Grade 3 to investigate the construct validity of CBM risk categories. The basic issue is whether the CBM measures assign children correctly to the basic risk categories of "At Risk," "Emerging," and "On Track." The data show that the CBM can be useful for identifying children at risk, but that the CI offers insight into the cognitive processes underlying student difficulties. The CI is particularly useful for identifying children whose underlying mathematical reasoning is more competent than their overt performance. Information of this type is more valuable for teachers than simply learning that a student is at risk.