## ← Point of SD - Intro to Descriptive Statistics

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Showing Revision 3 created 05/24/2016 by Udacity Robot.

1. >> Now you saw that it's pretty complicated to calculate the standard deviation,
2. but what's so great about it? Why don't we just find the average absolute
3. deviation? Why do we have to square each deviation, find the average square, and
4. then take the square root? Doesn't it seem like a lot of extra complicated
5. steps? Well actually, the standard deviation is really cool, and it helps us a
6. lot when we do statistical analysis. It turns out that with a normal
7. distribution, and if you remember, that's where the mean equals the median
8. equals the mode, right in the center of the distribution, which is symmetrical,
9. the standard deviation has great properties. Approximately 68 percent of the
10. data falls within 1 standard deviation of the mean. So here's 1 standard
11. deviation on each side, and I'm using the lower case sigma to represent standard
12. deviation. So 68 percent of the data falls between this value, which is x bar
13. minus 1 standard deviation, and this value, which is x bar plus 1 standard
14. deviation. And 95 percent of the data falls within 2 standard deviations. So
15. this value here is x bar, the mean, plus 2 standard deviations, and this value
16. is x bar minus 2 standard deviations. So 95 percent of the data will be between
17. this value and this value. In fact, we can approximate how much data will lie in
18. between any number of standard deviations from the mean. We know this because
19. some poor mathematician calculated this for us and put it all into a really nice
20. table. We're going to use this table a lot in the course, but not yet.