Reply to 2 standard deviations from what?

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Reply to "2 standard deviations from what?"

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[quote=Anonymous]When a test is designed, the creators "norm" the test on a (hopefully) large population of children, and that helps them to see the way that the scores should be interpreted. Standard deviation is a term that means that if you average all of the test scores of all of the children taking it, 50% will be above a point, and 50% below. That is your mean, and it is usually 100.

One standard deviation is 34.1% of the population both above and below the mean, and that is your average range-68% of the population falls in this group and is average. Usually this is 15, but sometimes 10. So a score of 85-115 is average if the SD is 15, and these scores account for 68% of the population.

Another SD adds 13.6% of the population; one above and one below the mean. Two SDs below the mean would be scores less than 70, and two above would be scores higher than 130. Scores between 84 and 71 are below average (more than one SD below the mean), scores below 70 are significantly below average (two SDs below the mean), and the same for above the mean. All the scores from 70-130 account for about 95% of the population. Therefore, only about 2.5% of the population would score more than two SDs below the mean, which is a rather rigid definition of impairment. Many school districts use 1.5 SDs.

https://en.wikipedia.org/wiki/Standard_deviation has pictures and more information.[/quote]

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