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How to calculate standard deviation (S1B)

I have my Statistics exam this afternoon (GCE AQA Statistics 1B) and I've totally forgotten how to calculate the standard deviation. How do you calculate it when you get given raw data and when you get values of like sigma x and sigma x^2? This would be really helpful for someone to give me a hand?
Reply 1
Avatar for Kat_Heine
Kat_Heine
you have you're random data such as... 5,7,8,8,8,8,9,12 - call this x

you want to find the mean of x (x'bar') so in this instance it would be 8.125

you then minus the mean from all the individual the x values and squared so: (5-8.125)^2 etc.

then calculate the total of (x-xbar)^2

divide by 'n' the number of 'x' values and take the square root for your answer.
Reply 2
Avatar for L O N D O N
L O N D O N
the standard deviation = sqaure root of the variance. or SigmaX^2/n - (mean)^2
Reply 3
Avatar for spliishie
spliishie
OP
7
Original post by HugoDuchovny
your supposed to input the data into your calculator
and use it to calculate s, as opposed to using the formula
it saves soo much time

do you not know how to do that?
if not, quote me, and i'll give detailed explanation


No I know how to do that but sometimes it doesn't give you enough information. Like instead of giving you the raw data, it gives you the sum of x and the sum of x squared and other stuff like that and I can't remember how to put that together to get the standard deviation. I know the mean is the sum of x over n
Reply 4
Avatar for spliishie
spliishie
OP
7
Original post by L O N D N
the standard deviation = sqaure root of the variance. or SigmaX^2/n - (mean)^2


Thank you :-) That's helpful :-) do you then square root it or leave it as it is?
(edited 12 years ago)
Original post by spliishie
No I know how to do that but sometimes it doesn't give you enough information. Like instead of giving you the raw data, it gives you the sum of x and the sum of x squared and other stuff like that and I can't remember how to put that together to get the standard deviation. I know the mean is the sum of x over n


If you have the sum of x squared and the sum of x:

The sum of x squared divided by n MINUS the mean squared
And then square root the answer.

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