Standard deviation
SAT glossary · Math
Definition
Typical distance from the mean. The exam never asks you to compute it — the formula is not on the reference sheet; it asks you to COMPARE two sets: more spread out from the center means larger SD. It has nothing to do with the mean's value: 10,10,10,10,10 and 8,9,10,11,12 share a mean of 10 but have SD 0 and ≈ 1.41.
Standard deviation in plain English
Standard deviation measures how far, TYPICALLY, the data points sit from the mean (the average) — a low standard deviation means the data huddles close together, and a high one means it's spread out wide. Good news: this test never asks you to actually calculate it by hand (the formula isn't even given on the reference sheet) — it only asks you to COMPARE two data sets and judge which one is MORE spread out. Standard deviation has nothing to do with what the mean actually equals: for example, 10,10,10,10,10 and 8,9,10,11,12 both have the exact same mean of 10, but the first set has a standard deviation of 0 (zero spread, every value is identical) while the second has a standard deviation of about 1.41 (some real spread around that same center).
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