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 files: the SC Department of Education assessment data (SC READY, ELA & Math, grades 3-8). These files report, for each school / grade / year:

• the percentage of students in each performance level plus the number tested, both real, reported figures, which we convert directly to counts (count = round(percentage × number tested)), and
• the group’s real, reported mean scale score.

How the percentile is estimated

South Carolina publishes the exact scale-score range for each performance level (e.g. for Grade 3 Math, Level 3 “Meets Expectations” is 438-543). 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 converted counts, anchored to the group’s real reported mean. 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.
• SC READY (ELA & Math) uses a vertical scale from 100 up to a grade-specific ceiling (825 in grade 3 rising to 950 in grade 8). South Carolina moved ELA to a new vertical scale in 2025 and Math to a new scale in 2026 (100 up to 400 in grade 3, rising to 450 in grade 8), each with new standards and new cut scores, so scores from before and after a change aren't comparable; the cut scores were also adjusted by a point in places in 2023. Each year here uses the cut scores the state printed for that year. The four levels are Does Not Meet, Approaches, Meets, and Exceeds Expectations, with Meets marking the state proficiency line. No statewide test was given in 2020 (COVID). South Carolina is compared at the school, district, and state levels (its districts do not map to counties).
• Science is a separate test (SCPASS), grades 4, 6, and 8, with its own scale per grade (grade 4 runs 1370-1530). SCDE publishes real per-school figures, including the actual mean and spread, not an estimate, but stopped posting school-level Science reports after 2018 -- so Science here covers 2018 only, while ELA/Math cover every year back to 2018.
• Charter schools appear as their own districts in the state files and are included in the school, district, county and statewide comparisons.

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