Why We Should Monitor Percentiles, Not Scale Scores

“Your child’s scale score went up by six points since the first diagnostic.”



Wait, is six points good? Was it enough to stay on track with her peers?

How can we know?

It turns out, the major diagnostic assessments (think i-Ready, STAR, MAP) report a national percentile along with the scale score, which shows how that scale score compared to all others who took the same assessment in the same window of time.

For reasons still unclear to us, the percentile tends to be the best-kept secret in the diagnostic report.

Teachers have been trained to look for the reported raw scale score for a given student and compare it to the previous scale score to find the difference. When the difference is positive, they celebrate.

The inherent problem in this is the lack of context. If I gain six points but most everyone around me gained twelve, I grew but did not keep pace. If this continues unnoticed, I may fall too far behind to be able to catch up.

While the celebration may feel good, it could also send a false signal to the student, parent, and teacher that all is well. If we know that a six-point gain also represented a decline of two percentile points, we can celebrate the learning that occurred but also consider how to tweak instruction to help the student gain lost ground.

Question: How can I get my faculty and parents to understand percentiles?

Answer: They already do.

At Tyler’s six-month-old visit, the doctor announced that his head measured 45 centimeters.

“Is that big or small?” Dad wondered aloud.

“It’s in the 95th percentile.”

“Oh, wow, Tyler has a big head!

Case in point. For most adults, percentiles provide immediate context no matter what is being measured.

Tyler’s father couldn’t conceptualize whether 45 centimeters was a lot or a little, but he intuitively understood the 95th percentile was well above average.

Technically speaking, it means Tyler’s head is larger than 95% of six-month-old babies.

And if a student’s scale score is in the 40th percentile, it means around 60 percent of students scored higher on the same assessment.

We also know if the student comes into the school year at the 40th percentile, we want him to leave at the 40th percentile or higher, indicating a year’s growth in a year’s time.

Our advice—use percentiles as the primary gauge of student growth. No scale scores (or lookup tables) required.

To learn more about how K12 Lift can help visualize your diagnostic assessments in the context of your state accountability system, click here to contact us.

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