Growth vs. Achievement Explorer

Achievement percentile: where a score sits compared with students in the same grade in the test's norm group. The 82nd means higher than about 82 of 100.

Growth percentile (SGP): how this year's growth compares with students who started at the same score. This tool uses a common set of cut points: 1 to 34 low, 35 to 65 typical, 66 to 99 high. Your state may use others. Neither number is proficiency, which is a fixed bar your state sets.

Normal ups and downs: two scores for the same student differ by chance. The shaded range is where about 9 in 10 retests of the same student would land (1.645 standard errors of a change), so a change inside it can be measurement error alone. It is wider near the middle and narrower near the ends. When the change is inside it, the tool does not color the growth percentile either, because that number comes from the same two scores.

Do the numbers line up? From last test's percentile and the growth percentile, the tool estimates where a student usually lands this year. If the growth percentile those two percentiles imply is more than 20 points from the one entered, the tool asks you to check the report instead of explaining the numbers.

Drift toward the middle: a very high or very low score is partly luck, so the next score tends to land a little closer to the middle. A typical grower from the 90th usually lands near the 85th; from the 10th, near the 15th. The tool only uses this explanation when the starting rank was well above or below the middle.

Adequate growth percentile: the growth a state says a student needs to reach or keep proficiency within a set time. Growth at or above it meets the target.

Assumptions: each score carries measurement error of about 0.2 standard deviations, and scores correlate about 0.8 from one year to the next. Real tests vary, so check your test's technical manual. Growth percentiles are built differently by each test, so compare numbers from the same report.