Portfolio assessment has become a widely cited technique for obtaining a rich demonstration of the skills that students have acquired, and it is often implied that portfolio assessment provides different types of information from that obtained from standardized tests. This paper provides support for the contention that portfolio assessments do indeed provide information about student capabilities that is different from that available from standardized tests. This support is given in the context of a portfolio assessment model called PASSPORT designed for use with students in grades 9 through 12. PASSPORT is a portfolio assessment system for collecting and presenting evidence of student knowledge and skills in the areas of English language arts, mathematics, and science. Students produce portfolios of their work through a loose set of guidelines. Each portfolio contains five work samples and a cover letter that explains why samples were included and what they tell about the student's capabilities. To compare the content assessed by PASSPORT and the American College Testing Program (ACT) Assessment, a multidimensional item response theory model (MIRT) was applied to the mathematics and language arts scores from the portfolio and the multiple choice test. The MIRT analysis was followed by a cluster analysis of similarities of the dimensions assessed by the items. Comparison of the content and cluster analyses shows clear differences in the types of skills and knowledge that are assessed. The portfolio provides evaluations of student performance on major writing tasks and on mathematics skills related to data analysis and problem solving, while the standardized test provides information about the details of the writing process and the rules for manipulating mathematical expression. (Contains two figures, six tables, and nine references.) (SLD)