Value-added is more than just a trendy phrase. Jonathan Crane from the Progressive Policy Institute, has a very-readable 7-page report on the promise of value-added testing for public schools. The most impressive part of his report is the discussion of teacher quality and value-added testing.
In a study of second to fifth grade students in Tennessee between 1991 and 1995, William L. Sanders found, with all other things being equal, the top one-fifth of teachers raised their students' achievement test scores 39 percentile points more than teachers of the bottom one-fifth. . . . A 39 percentile difference is enormous. It implies that a child who winds up in the bottom one-third of the distribution (say in the 30th percentile) after a year with a poor teacher, might have wound up in the top one-third (the 69th percentile) if they had been lucky enough to get an excellent teacher that year. What's more, these large effects were found in all kinds of classrooms with all kinds of students. . . . They helped struggling students as well as those who were excelling.