The system highlights the difference schools make to children's progress - whether that be positive or negative - by comparing pupils' test results at seven years old to their results at 11.
Individual pupil scores are then averaged and each school gets a value-added measure based around 100, where 100 denotes average progress. A score of 101 means on average each of the school's pupils made one term's extra progress. Below 100 means that pupils have made less than average progress.
But one headteacher who took part in the pilot project said that even value-added scores were unfair. She was given a low value-added score, despite having test scores above the national average.