Formative assessment, rephrased for clarity as informative assessment, is classroom activity—from quiz to question, discussion to observation, learning task to student feedback—that informs teachers about their students' learning and their own teaching. It is at the heart of teaching for long-term learning.

Robert Rothman (2006) describes the differences between the two major kinds of assessment, summative assessment and formative assessment.
Summative assessment is directed at answering a key question—Do you understand?—with a yes or no answer expected. The questions are asked at the end of a unit of instruction, and the results are often analyzed months after the students have moved to another grade. In formative assessment, the key question is, What do you understand? Both teachers and students pose this question on an ongoing basis and use the answers to adjust instruction and learning strategies accordingly. As Stephen and Jan Chappuis tell us,

Even though assessments will continue to be labeled formative or summative,
how the results are used is what determines whether the assessment is formative or summative. (p. 15)

It is this formative assessment that research concludes has a significant effect on student achievement, according to Dylan Wiliam. Wiliam references five reviews of research synthesizing a total of more than 4,000 studies undertaken during the last 40 years. Their conclusion is clear:

When implemented well, formative assessment can double the speed of students' learning. (p. 36)

Thomas Guskey (p. 27) reiterates the point, "Done properly, formative assessment can generate dramatic improvements in teaching and learning." But, he emphasizes, what happens afterward—how the teachers use the information gleaned about both the teaching and the learning—is the rest of the story.

Scherer, Marge. Perspectives / An Answer for the Long Term. Educational Leadership. December 2007-January 2008. Volume 65. Number 4.

Formative Assessment Tool: Achievement Series