Certainly, the assumptions behind grading and testing are that student ability can and should be measured by the sum of the scores received on individual tasks or

assignments. Portfolios, on the other hand, provide the student and the teacher with a variety of writing samples that can only be understood and evaluated in context and relationship with each other. A judgment based on a student's portfolio can radically differ from a judgment based on an individual student text because it includes a range of contextual factors including but not limited to the other texts in the portfolio, the act of selecting pieces for inclusion, the act of writing about texts, and the process of writing and compiling the portfolio. The variety of texts within a portfolio exemplifies the progressive, developmental, and fluid nature of written language acquisition.  The texts in a portfolio typically devoted to reflection and writing about writing focus not only on the product of writing but on the process as well, illustrating what the student writer knows about the product and process of writing within his or her own experience as a writer...

[T]he way portfolios are used is a key feature for harnessing their potential. For example, it is possible to use portfolios within the same theoretical framework that underlies testing and grading by continuing to assign separate numbers or letters to individual papers within a portfolio. This is a common practice, because while it is relatively easy to switch to portfolios it is much more difficult to alter the assumptions behind our practices. If, however,we want to conceive of portfolios as a viable way to improve the way we assess student writing, then we need to consider them as discrete units to which we can assign value in their entirety.