I'm having a difficult time understand what these statistics functions do and how they work. I'm having an even more difficult time understanding how stdev works vs stdevp and the var equivelant. Can someone please break these down into dumb for me?
A:
STDDEV is used for computing the standard deviation of a data set. STDDEVP is used to compute the standard deviation of a population from which your data is a sample.
If your input is the entire population, then the population standard deviation is computed with STDDEV. More typically, your data set is a sample of a much larger population. In this case the standard deviation of the data set would not represent the true standard deviation of the population since it will usually be biased too low. A better estimate for the standard deviation of the population based on a sample is obtained with STDDEVP.
The situation with VAR and VARP is the same.
For a more thorough discussion of the topic, please see this Wikipedia article.
jbourque
2009-09-21 16:31:17
I'm not sure about SQL, but this doesn't seem to agree with the Excel definitions of STDEV and STDEVP. The Wikipedia article explains how the population standard deviation is underestimated by calculating the actual standard deviation of the sample (divide n), and thus that it is usually corrected (divide n-1). STDEVP is (n) [http://office.microsoft.com/en-us/excel-help/stdevp-HP005209281.aspx?CTT=1] and STDEV is (n-1) [http://office.microsoft.com/en-us/excel-help/stdev-HP005209277.aspx?CTT=1].
Ben Challenor
2010-07-05 20:01:42