Dublin teacher, Kevin Griffin, brings to our attention this graph, which he describes thusly
If the theory behind evaluating teachers based on value-added is valid then a “great” 7th grade math teacher should also be a “great” 8th grade math teacher (upper right corner) and a “bad” 7th grade math teacher should also be a “bad” 8th grade math teacher (lower left corner). There should, in theory, be a straight line (or at least close) showing a direct correlation between 7th grade VA scores and 8th grade VA scores since those students, despite being a grade apart, have the same teacher.
Here's the graph
Looks morel ike a random number generator to us. Would you like your career to hinge on a random number generator?