Differences in STDEV and STDEVP

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Differences in STDEV and STDEVP

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SamC
White Water Yakist

3467 Posts

Posted - 2005-05-03 : 15:06:44

BOL isn't much help. I probably missed the boat again.

SamC
White Water Yakist

3467 Posts

Posted - 2005-05-03 : 15:10:30

I found this. I've got to go back to my stats book now.

STDEV vs. STDEVP

As mentioned earler, standard deviation mainly comes in two flavors:

* STDEV is used when the group of numbers being evaluated are only a partial sampling of the whole population. The denominator for dividing the sum of squared deviations is N-1, where N is the number of observations ( a count of items in the data set ). Technically, subtracting the 1 is referred to as "non-biased."

* STDEVP is used when the group of numbers being evaluated is complete - it's the entire population of values. In this case, the 1 is NOT subtracted and the denominator for dividing the sum of squared deviations is simply N itself, the number of observations ( a count of items in the data set ). Technically, this is referred to as "biased." Remembering that the P in STDEVP stands for "population" may be helpful. Since the data set is not a mere sample, but constituted of ALL the actual values, this standard deviation function can return a more precise result.

Look at the example again, comparing Rows 18 and 21, to see how these calculations return two slightly different standard deviation values for the same data. Notice that the % Err of Mean is lower for STDEVP, which assumes we are evaluating the entire known population.