Standard Deviation

Hi there, I am studying (self-teaching) a geography a-level and had a question(s) about standard deviation. I felt as though perhaps the maths forum was the best to post it on as I found it much more of a maths based question(s). So I've got a table of rock sizes from 8 sites. There are 15 different rock sizes from each site so I have calculated the mean of these 15 rock sizes and have, for example 9.033 and 7.92 etc. etc. So I presume that these numbers will go in the 'x' column of the standard deviation table? ^^^ first question I have added all these numbers up, found the mean etc. for the next 3 columns and got a result of the alien E thing to be 47.2 (that seems about right for my 8 result numbers). Now for the second question it is about calculating the n. So I have the 15 (clasts/rock sizes) but only 8 results. So I presume that n = 8 and not 15? Also do I have to do n-1? In my revision book it just says to divide E by N and square root it; however on the internet it says to divide it by n-1? I'm confused. I do not have any teachers etc. to ask as I am self-teaching at home so any help for either of the questions would be much appreciated! Thank you!

I don't know what book you are using but this is the formula you should use (but change N to N-1, since its a sample). The table is a way of breaking this down.

First work out what you want the standard deviation of i.e standard deviation of rock sizes from site A
Write all the rock sizes in the x column, then work out the average of these (this is Xbar), put this in the next column.
Then do a column of (X-Xbar), then write a column squaring these values. Then total up that column and divide by the number of Xs in your sample minus 1. Finally take the square root of this value and you're done!





Hope that kinda makes sense.

I don't know what book you are using but this is the formula you should use (but change N to N-1, since its a sample). The table is a way of breaking this down.

First work out what you want the standard deviation of i.e standard deviation of rock sizes from site A
Write all the rock sizes in the x column, then work out the average of these (this is Xbar), put this in the next column.
Then do a column of (X-Xbar), then write a column squaring these values. Then total up that column and divide by the number of Xs in your sample minus 1. Finally take the square root of this value and you're done!




Hope that kinda makes sense.

This is correct & i assure u this explanation should help. If u want any help msg me and I can help xx