How we estimate percentiles

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 official public test-results data published by the Illinois State Board of Education (IAR: ELA & Math grades 3-8; ISA: Science grades 5 & 8). For each school / grade / year:

• Every subject and year: the state's own reporting site publishes the real number of students tested and the real number in each performance level, for every school, district, and the state. We use those counts as published; no modeling of the counts themselves is needed, only the mean (see below). Illinois doesn't publish a mean scale score for any subject or year.

How the percentile is estimated

Illinois publishes the exact scale-score range for each performance level (e.g. for Grade 3 Math in 2024, Level 5 “Exceeded Expectations” is 790-850; in 2025 the same grade's top level, “Above Proficient,” is 781-850). Given your child's score, we:

1. identify which performance level the score falls in;
2. count everyone in the lower levels as scoring below;
3. 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. Students bunch up toward the middle of the scale, so an even spread would put too many of them out at the far ends of the lowest and highest levels. Checked against published statewide score distributions, this lands within about one percentile point of the true value for a typical score.

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 group's real counts, anchored to a mean we derive from those counts in every subject (Illinois doesn't publish a mean scale score for any subject). 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.

When a group's students all sit in one or two levels, the counts can't say how spread out their scores are, and the fitted curve can come out far too narrow. Those curves are drawn no narrower than one fifth of the statewide spread, which is about the size of the test's own measurement error, and inside the student's level the percentile spreads that level's students evenly instead.

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 counts, so their estimates are noisier.
• Some school/grade cells are suppressed by the state (for privacy, small groups) and won't appear.
• IAR reports a scaled score from 650 to 850. Through 2024 it had five levels (Level 4 = Met, Level 5 = Exceeded Expectations) with the lower cuts fixed at 700/725/750 every grade. No statewide test was given in 2020 (COVID).
• ISA (Science) reports a scaled score from 700 to 900. For 2022 through 2024 it had four levels named Emerging, Developing, Proficient and Exemplary.
• 2025 changed the performance levels for every subject. Illinois re-cut all of its tests onto one common set of four levels (Below Proficient, Approaching Proficient, Proficient, Above Proficient) and raised the proficiency bar, so ELA and Math went from five levels to four and Science kept four levels under new names and higher cuts. The scales themselves did not change. Percentiles stay comparable across the change because each year places a score against that year's own cohort, but a level label from 2025 does not mean the same thing as the label a child got in 2024.
• Charter schools appear as their own districts in the state files and are included in the school, district and statewide comparisons.

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