I have another question on the flashcards - I've attached the document, but in a nutshell, the question is - are there more than one SD formula?

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SD RMS question.doc (28.0 KB, 17 views)

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You divide by N for a population standard deviation, and by N-1 for a sample standard deviation. Take a look here for further explanation. I don't remember this being a cause of confusion when I took the ECE -- seems like knowing that the SD was the square root of the variance was enough for most questions, but don't trust my memory.

-Gary-

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I see where you got confused because I got stuck at the exact same place!! (I even posted about it!) You only do the n-1 when your SD is for a sample (as opposed to a population).
Does your calculator do this for you? Mine does, I just have to enter the data set, and then select which key I want depending on if I want it to calculate SD of a sample or SD of a population. Check your calculator, it can make your life easy. I worked out like 40 of these long hand before I found my magic button

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