Highdesertmike
Active Member
Yes, it is a statistical "window" of how close the shots will fall within the group. Here's a simple explanation from TechBookReport.com:
Standard Deviation In 30 Seconds
(Or standard deviation for dummies...)
What is it?
The standard deviation is a measure of statistical dispersion.
In plain English it's a way of describing how spread out a set of values are around the mean of that set. For example, if you have a set of height measurements, you can easily work out the arithmetic mean (just sum up all the individual height measurements and then divide by the number of those measurements). However, knowing the mean (or average, as it's more commonly called), doesn't tell you about the spread of those heights. Were all the people in your group the same height, or did you have some tall and some short, or was there one really tall person who towered over everybody else? It's possible that you could have exactly the same average height from wildly different groups. Knowing about how spread out those heights are compared to the mean gives you extra information over and above the mean value.
How do I calculate it?
The basic idea of the standard deviation is that you're measuring variations around the mean value. Some of those values will be below the mean, some above and sometimes you'll have some that are equal to the mean. In other words some of the differences between the individual measurements will be positive (more than the mean), some will be negative (below the mean) and some will be zero (directly equal to the mean). Now just adding these differences up is dangerous because the positive and negative values will cancel each other out. For example, to take an incredibly simplistic case, if you've got a sample of two values, one of 9 and one of 11, the mean is equal to 10. The differences are -1 and +1, adding these together gives us a total variation of 0. But we know that there's not zero variation around that mean value!
So, to get round this problem each of the variations around the mean is squared. When you square a negative value you get a positive value. So, to work out the standard deviation we square all of the differences from the mean, add them all up and divide by one less than the number of values in our set. This new number is called the variance. Now we take the square root of the variance (which is reversing the squaring we did earlier so that our number is closer to the original differences), and that's the standard deviation.
