Student achievement as reflected in standardized test scores is not a unitary construct, but a set of interdependent components representing degrees of knowledge of various subject areas. The interdependency of achievement components imply a positive feedback process. Weaknesses in one area reinforce weaknesses in others; strengths in one area reinforce strengths in other areas. Structures of achievement battery component scores were related to ethnic group differences by using discriminant analyses of Iowa Tests of Basic Skills major skill area scores from each of six grade levels. Vocabulary was identified as a key component of achievement related to ethnic group differences, reflecting the persistent handicaps experienced by black and Mexican American students. Mexican American students seemed strong in math relative to black and white students when linguistic differences were controlled. The findings suggest a need for further investigation in the area of ethnic group differences. (Author/KMF)