Observers rarely used the top or bottom categories (“unsatisfactory” and “advanced”) on the four-point
observation instrument, which was based on Charlotte Danielson’s Framework for Teaching. On any
given item, an average of 5 percent of scores were in the bottom category (“unsatisfactory”), while just 2
percent of scores were in the top category (“advanced”). The vast majority of scores were in the middle
two categories, “basic” and “proficient.” On this compressed scale, a .1 point difference in scores can be
sufficient to move a teacher up or down 10 points in percentile rank.