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## ← SD of Karma Points per Post - Intro to Descriptive Statistics

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Showing Revision 4 created 05/25/2016 by Udacity Robot.

1. This one probably took you a little longer to figure out, and that's okay. So,

2. you remember that the first thing we have to do is find the deviation from the
3. mean for each of these values. So, let's do it in cell B1. We found that the
4. mean was approximately 12.96. So, I'll write equals A1 minus 12.96. And that
5. will give us the deviation of the first value from the mean. Now, we'll copy
6. this so that we don't have to write it in every cell. And then, if you go to
7. cell A1 and push Cmd+Down, at least on a Mac keyboard, and then we'll go to cell
8. B200 and then push Cmd+Shift+Up Arrow key, then you should have the whole column
9. B highlighted. Now, write Cmd+V and then you have all of the deviations for each
10. of those values. Something else that you may have done in which I taught you
11. before, was going to this corner until you see the plus and then dragging it
12. down. And if you did that, that's perfectly fine. These are just some keyboard
13. shortcuts that make our life a little easier. So, in column C now, let's square
14. each of the deviations, so we'll write equals B1 carrot 2, which means squared,
15. and then again, we'll Copy cell C1, go to cell B1, push Cmd+Down Arrow key, to
16. go to the bottom. Go to cell C200, push Cmd+Shift+Up Arrow key, so that we have
17. all of column C highlighted, and then, Cmd+V. And now, we have all of our
18. squared deviations. So, what's the next thing we do? We have to sum them up. So,
19. let's do that in cell D1, equals sum of C1 colon C200. So, we added up our score
20. deviations and now this time, and this is where the sample standard deviation
21. differs from the population standard deviation, we'll divide this by n minus 1,
22. which is 199 equals D1 divided by 199. And then, our last step is to take the
23. square root, equal sqrt of D2. So, we get that our sample standard deviation is
24. approximately 4.76, and this is our best estimate of the population standard
25. deviation.