Dublin teacher, Kevin Griffin, brings to our attention this graph, which he describes thusly

The chart plots the Value-Added scores of teachers who teach the same subject to two different grade levels in the same school year. (ex. Ms. Smith teaches 7th Math and 8th Math, and Mr. Richards 4th Grade Reading and 5th Grade Reading.) The X-axis represents the teachers VA score for one grade level and the Y-axis represents the VA score from the other grade level taught.

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?