The scale is basically a [normal distribution](https://en.m.wikipedia.org/wiki/Normal_distribution) centered around an average of 100. Every 15 points is one standard deviation. 85-115 holds about 70% of the population. 70-130 covers 95%, and 99.7% of the population falls between 55 and 145. At least, that’s how it was intended. The average test score has shifted over time (generally but not always up). Usually when a new test is created, the scores are standardized as above, but the same people taking older tests will average out to greater than 100.
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