The effectiveness of portfolio assessments in science
Slater (1997)

what was studied
Slater examined the effectiveness of different types of portfolios in three separate studies: (1) a college physics course at an urban community college; (2) a course in physical science for elementary education majors at a medium-sized university; and (3) an large introductory environmental science course for non-science majors at a major university.

how effectiveness was measured
Each study compared two groups (one assessed by portfolio, the other assessed by more traditional means such as tests and quizzes) on the following measures: a common final examination, a pretest/posttest self-report survey, open-ended surveys, and focus group interviews. Portfolios were evaluated with a holistic scoring rubric^{41, 45} at regular intervals throughout the course.

what the findings were
Students assessed by portfolios scored just as well on a traditional multiple-choice final examinations as students who were assessed by more traditional means. Student surveys and focus-groups revealed that portfolios reduced test anxiety and students' focus on note-taking rather than participation during class discussions. Students assessed by portfolios also reported that they spent substantial time reviewing the textbook to make sure they understood the content deeply, thought about course content outside of class, and felt they would remember the course material better than the material from other courses given the time and energy they invested in creating their final products.