This TED presentation says that the New York Department of Education is using a secret algorithm to evaluate teachers:
(..) teachers were being scored with a complex, secret algorithm called the "value-added model (..)
I have looked for this algorithm and there is some criticism surrounding it (Huffington Post):
An algorithm is an automatic way to determine results. No need to have those pesky human factors like judgment, emotion, discernment, understanding, empathy, sympathy, critical reasoning involved.
While using or not an algorithm for evaluation is clearly debatable, I am interested mostly in the not being public part. This sounds strange as teachers do not know what they should improve if they are penalized by it.
Question: Why was the "value-added model" algorithm kept secret?