How we estimate percentiles

Plain-English 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 3-7; ISA: Science grades 5 & 8). For each school / grade / year:

ELA and Math (IAR): the bulk Report Card file gives the percentage of students in each performance level per school/grade/subject, with no separate tested count. We treat these published percentages as a group of 100 students to preserve their shape, and derive an implied count scale, mean, and spread from them using the same statistical fit described below.
Science (ISA): the state's own reporting site publishes real counts — the actual number of students tested, and the real number in each of the four achievement levels, for every school, district, and the state. We convert those to percentages the same way as every other level-count state; no modeling of the counts themselves is needed, only the mean (see below).

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

How the percentile is estimated

Illinois publishes the exact scale-score range for each performance level (e.g. for Grade 3 Math, Level 5 “Exceeded Expectations” is 790-850). Given your child's 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, use linear interpolation across that level's count, assuming scores are spread 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 group's counts (real for Science, percentage-derived for ELA/Math), 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 headline percentile comes from the counts (step above), not from the curve.

Why estimates are still useful: for Science, the counts are real and complete; only the mean is derived, the same well-grounded modeling every state in this tool uses. For ELA and Math, the performance-level percentages are real for each group, but because those files carry no separate tested count, both the implied student count and the mean shown are derived from the percentages alone. Either way, the same consistent method is applied across every school and grade, so the comparison (“above/below average here vs. statewide”) holds up well even though ELA/Math figures carry more modeling than most other states in this tool.

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 with five levels (Level 4 = Met, Level 5 = Exceeded Expectations); the lower cuts are 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 with four levels (Emerging/Developing/Proficient/Exemplary), covering 2022 through 2024 (the achievement-level cuts changed for 2025-26, so more recent years aren't included yet).
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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