A positive relationship between statistics achievement and mathematics competence has been consistently reported in literature. Given the influence that mathematical ability has on statistics achievement, it turns out to be useful to assess the mathematical competence in order to promote statistics course performance. In this work, we developed a scale to measure the mathematical ability deemed necessary for psychology students to successfully complete an introductory statistics course. The Item Response Theory was applied to construct the instrument in order to overcome the limitations of the classical approach. The predictive validity of the instrument was assessed considering achievement as the criterion measure. Advantages offered by using the scale in introductory statistics teaching are discussed.