School choice supporter Lance Izumi from the Pacific Research Institute for Public Policy offers this:

Talking to National Public Radio, Melody Barnes, a spokeswoman for [Barack Obama's] campaign, said recently that Mr. Obama supports "portfolio assessment" of student performance. Portfolio assessment usually requires a student to perform various classroom assignments, like write essays, do individual projects, participate in group projects. These assignments are put into a portfolio for that student and evaluated. In a debate earlier this month, Linda Darling-Hammond, education adviser to Mr. Obama, pointed to other countries where students are assessed based on “kids doing science inquiries, research papers, technology products."

Portfolio-assessment supporters claim that this method gives a broader view of a student's knowledge level. Whether this is true or not, the problem is that portfolio assessment is extremely difficult to implement effectively in practice.

Kentucky enacted a portfolio-assessment program in 1990. After a few years, a state review panel found that because teachers in the students’ own schools scored the portfolios, there was grade inflation. Furthermore, the panel found that since teaching, time allotted for assignments and opportunities for revising work differed from classroom to classroom, it was impossible to compare student performance fairly.

A RAND study of Vermont’s portfolio-assessment system, which used independent scorers, came to similar conclusions. The study found that those assessing portfolios were confused by the scoring guidelines and disagreed among themselves during the evaluations. Moreover, training a large number of scorers proved difficult and the system was hugely expensive. Finally, student task variation from classroom to classroom made reliable evaluation impossible...