The importance of critical thinking in education is underpinned by decades of theoreticaland practical work. Differences have been demonstrated between students who receiveeducation in this area from an early age and those who do not. However, not enough workhas been done to measure these skills in a classroom setting. Given that the best time toteach critical thinking is during the first years of primary education, we designed a testto determine the level of critical thinking among 3rd and 4th grade students in LanguageArts using a graphic novel. We showed the legitimacy of the instrument through ConstructValidity, Content Validity, Pearson product-moment correlation, Reliability and Item Anal-ysis. Using the instrument, we studied how critical thinking skills differ among 3rd gradestudents, according to their socioeconomic status (SES), studying schools with low, middleand high SES. We found significant differences between the schools, which suggest thatthere may be a relationship between socioeconomic status and the development of criticalthinking skills. The test presented in this study is an improvement on existing assessmenttools as it replaces unidimensional models with models that provide a more detailed andmultidimensional picture of student learning.