Do you know how to calculate the standard deviation for a normal set of data?
Ok... that's not what I meant. That's using your calculator mindlessly to get something you're told is the standard deviation. I'm asking if I gave you a set of data and a calculator that could only do addition, subtraction, multiplication, division, squaring, and square roots... could you calculate the standard deviation of the set of data?
To put it another way: You're relying way too much on your calculator. If you take the time to think about what you're actually doing and what you're supposed to be doing you WILL save yourself a lot of time and hassle in the long run. By cheating yourself and just pressing a button on a calculator you clearly don't understand what's actually going on. Learning what happens behind the scenes when you press that button is what you need to know how to do - that's what is important here.
newbiestats (11-12-2011)
I think we should step back and explain a little theory here, because newbie doesn't seem to be getting exactly what went on in the mean example.
You should be aware that the mean is simply the sum of each data point divided by the number of points in the data set. Right?
With data from a frequency table, we don't have the exact values. Instead, we know the total count. It is the sum of the frequency in all the bins (= 212). To approximate the values, we take the midpoint of each bin to represent the overall value. For instance, a bin of 30 to 40 with frequency 10 might have values 30, 30, 31, 34, 36, 38, 40, 39, 35, 35. We don't know the details. Instead, we simply say the midpoint is 35 and in our ignorance use that to represent all the values in the bin. Thus, we would have 35 times 10 as the sum of data points from that class.
In the example you provide and the way Dragan showed how to do it, we had: , for j different classes. Thus, we were simply calculating the mean as always. So how do we calculate standard deviations? You'll need to create your frequency data set, using the "frequency x midpoint" to generate a frequency-sized data set for each class. That is your input. I'm sure your calculator has some way of doing that work for you, but this is ultimately just arithmetic you should be able to do yourself, without any tools.
newbiestats (11-12-2011)
Finally I figured it out, thank you all very much . Here is what I did, in my calculator I went on sd mode, then I entered the data other way round, I did like this:
30 SHIFT , 30 M+ and 45 SHIFT , 20 ..... but before I entered the data as this: 30 SHIFT , 30 M+ and 20 SHIFT , 45, hence i was getting the wrong answer because I was putting it the other way round in the calculator. Thanks you all once again
Technology FTW!
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