These differences result in important variation in student performance across the two assessments. The two assessments also differ in item formats - in their balance of multiple-choice vs. open-ended responses. Most importantly, however, the two assessments differ in their framew'ork and content, that is, in what they are seeking to measure. In contrast, the objective of PISA is to measure the skills and competencies students have acquired and can apply to real-world contexts by age 15.