Posted: 4/23/2006 11:06:45 PM EDT
| what is it and why is it important to me? |
Standard deviation is the summation of the squared deviation of every data point from the group mean, divided by the group size (or one minus the group size if it is sample data, and not population data), and then square rooted. The formula looks like this:  What it tells us is about how much variation there is from the mean for normally distributed data. The rule of thumb is that about 2/3rds of your data will fall within +/- 1 standard deviation, 95% within +/- 2 standards deviations, and nearly 100% within +/- 3 standard deviations. In other words, If my mean is 3000, with a standard deviation of 20, about 67% of my population will fall between 2980 and 3020 (+/- 1 std. dev.), 95% will be between 2960 and 3040, and virtually all examples will fall between 2940 and 3060. Hope this helps. 
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A bell curve is used to shows how many std. deviations you are from the mean. As PAEBR already stated, the standard deviation will tell you the probability of your value falling within a certain distance from the mean. The middle of the bell curve is the mean. You move away from the middle as you account for more of the population being studied. The equation above is how you mathmatically calculate your standard deviation, provided you have the population stats and the mean. |
| THat is an odd result. SD of velocity is of course only one part of a complicated dynamic. That being said, velocity differences should be directly proportional to vertical group size. One might not notice the difference at under 100 yards, but a tight velocity SD will definately help if say shooting steel rams at 200 yards. |
So "1.75 inches" is for parlor amusement?  |
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NVGdude: Good point, I was only testing at 25 yards and have not tried extended ranges on paper. bnorman: Yes 1.75" is not good at 25 yards from a sandbag! Eventually 3/4" was the best I got. But that was a One Time group I have never been able to reproduce… I didn't keep the target so it remains a fish story. 1 1/4" seems the best I can do but there is often a 2" or 3" unexplained flyer to crush my ego. (10mm Delta Gold Cup) |