The Shadows of Value-Added

In part due to its embrace of President Obama's Race to the Top, the state of Ohio will be using value-added testing metrics to evaluate how well its teachers are performing.  An interesting detail?  The formula used to calculate teaching effectiveness will be hidden from the public (emphasis added):

Educators have long argued that it would be unfair to judge teachers based on test scores, because students all start the year with varying levels of knowledge. A teacher who has a class full of students who are all two grade levels behind in math could be strongly effective, yet all that teacher's students may still fail state standardized tests.
Value-added tries to rectify that bias by measuring student growth over the school year, not just their final scores. Battelle for Kids, the company contracted by the Ohio Department of Education to develop the state's value-added system, develops a formula that measures average growth for a group, say fourth graders in Ohio. The contents of that formula aren't public.