Standard Deviation - Maths Help needed

OK I haven't learned about this for years since I was in high school, so can someone give me a hand. I've started back at Uni and I've been given 24 hours to finish off a Lab report but I'm having major calculator troubles... I can't remember how to use it.

I have a vague idea on what SD is, but I'm not 100% on it. Can someone work out for me what the SD for these numbers are?

140, 90, 66, 60, 69, 60. I've already got the mean as 80.8 and the SD I have is 28.3. Is that right? And what does that number mean?

 

The standard deviation shows the range of scores in a way. Say your mean is 30 and your SD is 10.

10 20 30 40 50

30 Is your mean. 68% of the scores will fall within one SD, so its 34% between 20 and 30, and another 34% between 30 and 40.

Then it's another 13.5% between 10 and 20, and 40 and 50, meaning that in total, 95% of all total scores will fall within two standard deviations (of 10) from the mean.

The formula for group data looks as such, it will work for regular data, just takes out the FI stuff

xi|fi|xi(fi)|mean|xi-mean|(xi-mean)²|fi(xi-mean)²


SD =
SQUARE ROOT OF THIS WHOLE THING:

fi(xi-mean)²
_____________
N <---The number of scores total you have.

Again for non group data take out all of the FI's, and just take the Square Root of (xi-mean)² over N.

 

You have 2 choices...the first is to use the stdev formula in Excel, and get the computer to do it.

The second is to go by the formula....unfortunately, though, I cant type it here...it doesnt want to work out.

But for me, it comes out as 31.02?

 

OK, so I've done it the long way by square rooting all of the mean minus score results and then dividing up the total by the number of scores. I got 28.3. How's that sounding?

 

Here is the SPSS output for your data set.

I too got a diffrent standard deviation from you

You might have to drag and drop this link

 

I too have been getting 31 today as well. Should I be using the XOn button or the XOn-1 button on the calculator?

I hit the XOn and got the 28, but the XOn-1 gives me the 31.

 

n-1, Wedge...its about degrees of freedom.

The equation with n results in sample biases of some sort or another.

I'd like to help more, but I haven touched statistics seriously in over 6 years....

 

If I remember correctly, The use of Xon is when that is the whole population, and Xon-1 is used when it is a sample.

Lord_Riven