WEBVTT

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Here are eight values: two, four, four, four, five, five, seven and nine.

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We will find the centre and the typical distance from it.

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The mean is the sum divided by the count.

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The values add to forty, and forty divided by eight is five.

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Subtract the mean from each value and square: nine, one, one, one, zero, zero, four and sixteen.

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These squared deviations add to thirty two.

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If these eight values are the whole population, divide by eight: the variance is four and the standard deviation is two.

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If they are a sample, divide by n minus one, which is seven, giving about two point one four.

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To recap: find the mean, add up the squared deviations, divide by n or n minus one, and take the square root.