http://shankerblog.org/?p=6265
Example: Imagine 4 students scored on a 10-point scale where 5 and above is proficient. Year 1 the scores are 4, 4, 4, and 10, resulting in 25% proficiency and an average score of 5.5. Year 2 the scores are 5, 5, 5, and 5, resulting in 100% proficiency and an average score of 5. According to this analysis, for ELA and math students in New York this kind of phenomenon is happening 15-20% of the time.