Effective differentiation requires teachers to assess student status before a unit of study begins (pre-assessment), throughout the unit of study (formative or ongoing assessment), and at key ending or wrap-up points in a unit of study (summative assessment). Pre- or diagnostic assessment helps determine a student's starting point with learning targets (KUDs) as well as with prerequisite knowledge, understandings, and skills that are essential to continued progress in a content sequence. Pre-assessment is also useful in developing awareness about students' interests and learning preferences. Formative (ongoing) assessment lets teachers closely monitor a student's evolving knowledge, understanding, and skills—including any misunderstandings a student may have or develop about key content. As with diagnostic or pre-assessment, formative assessment also plays a role in revealing students' various interests and approaches to learning. Summative assessment evaluates a student's status with the learning targets or KUDs at designated endpoints or checkpoints in a unit of study—for example, at the end of a section of a unit, end of a marking period, end of a semester, midterm, and so on.
http://www.ascd.org/publications/books/108028/chapters/Asses...