How we estimate percentiles: Iowa high school

A clear explanation of the data and the method, so you can judge how much weight to give the numbers.

What the data is

Every number here comes from the Iowa Department of Education “Proficiency by Public School” and “Proficiency by District” files and the Iowa School Performance Profiles, for the Iowa Statewide Assessment of Student Progress (ISASP) that 9th, 10th and 11th graders take in ELA and Math and 10th graders take in Science, for 2019 and 2021 to 2025. They report:

for every school and district, the real number of students who are proficient and not proficient, and the number tested. Iowa’s public files don’t split proficient students into Proficient and Advanced, so that split is estimated from the group’s average score. Statewide, Iowa publishes all three levels (2019, 2024, 2025) or the proficient count (2021-2023). A county is the sum of the schools in it, and
Iowa’s own average scale score for each district, and for each school in 2024 and 2025 (for 2019-2023, each school’s published index is converted back to an average score using Iowa’s statewide average and spread). Where neither exists (school Science in 2022 and 2023), the average is solved from the proficiency rate. The state’s average is Iowa’s published statewide figure from the ISASP Annual Statistical Report; a county’s is estimated from its level counts.

Who is counted

The public school students in grade 9, 10 or 11 who took the ISASP in that subject at that school that spring, including students who took the Dynamic Learning Maps alternate test. Iowa leaves out English learners in their first or second year in U.S. schools. Groups of fewer than 10 students are not reported, so some small schools do not appear for some years. A percentile compares a score with that year’s takers of the same test in the same grade.

Grade 9 starts in 2021: Iowa’s 2019 files have no grade 9 results for schools or districts. Grade 10 Science starts in 2022, the first year Iowa’s school files include Science.

Important: the public data does not contain individual student scores, only these group summaries. So no tool can compute a true rank of one student against every other student. What we provide is a well-grounded estimate.

How the percentile is estimated

Iowa publishes the exact scale-score range for each performance level (for example, grade 10 Math is Proficient at 537-653). Given a score, we:

identify which performance level the score falls in;
count everyone in the lower levels as scoring below;
within the score's own level, spread that level's students along the group's fitted curve (below) rather than evenly across the level's range.

That gives the percentile: the estimated share of students in the group who scored at or below the entered score.

About the smooth curve

The bell-like curve on each chart is a truncated-normal distribution fitted so that its area within each performance level matches the level counts, centered on the group’s average score described above. It’s there to visualize the shape of the group’s scores. The level counts set the headline percentile (steps above); the curve only decides where inside the student’s own level the line falls.

Why estimates are still useful: the proficient and not-proficient counts are real and complete for each school and district; only the split between Proficient and Advanced is estimated. Across a whole school, district, county or the state, the remaining assumption (how scores are spread inside a level) averages out well, so the comparison (“above or below average here versus statewide”) is reliable even though the exact percentile is approximate.

Cut scores

The score range for each level is on the score guides. Scores in Proficient and Advanced meet Iowa’s standard. The three ranges are Not Yet Proficient, Proficient and Advanced, the same in every year from 2019 to 2025.

ELA: grade 9 410-504, 505-617, 618-750; grade 10 435-529, 530-641, 642-780; grade 11 460-560, 561-659, 660-800.
Math: grade 9 410-512, 513-625, 626-750; grade 10 435-536, 537-653, 654-780; grade 11 460-558, 559-674, 675-800.
Science: grade 10 435-544, 545-655, 656-780.

The ranges are from Iowa Testing Programs’ “Cut Score Ranges for Performance Levels” (approved by the Iowa State Board of Education in September 2019). The 2025 ISASP Annual Statistical Report labels every scale score with its level, and those labels match these ranges. Iowa set new ranges for 2026, which are not yet published, so 2026 is not shown yet.

The high-school progress report

The paid progress report puts every high-school test your child took in one table, with the percentile at each level from the school to the state, and charts each subject across the years.

Lines join only the same kind of comparison. Grade 9, 10 and 11 ELA and Math, and grade 10 Science, continue the grades 3-8 ISASP lines of the same subjects.
Earlier years are compared statewide only. You can add your child’s grades 3-8 scores in the same subjects. The tool doesn’t know which school or district they attended, so those years are compared with every student in the state in that grade, not with this high school or its district.
Compare percentiles, not scores, across grades. Each grade has its own score range, so a percentile is the fair way to follow a student from one year to the next.

Limitations to keep in mind

Percentiles are estimates, not official rankings. Treat a “62nd percentile” as “roughly the top 40%,” not an exact standing.
Small schools have small groups, so their estimates are noisier.
The split between Proficient and Advanced is estimated for every school and district; the other counts are Iowa’s.
No statewide test was given in 2020 (COVID).

This site stores nothing you type. Scores are used only in your browser to draw your charts.

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