The Firing Line Forums - View Single Post - What is the signifigance of Standard Deviation??

mongoose33 · March 11, 2010, 01:02 AM

Standard deviation is simply a way to quantify spread of a series of data points, like velocities.

It's better than the extreme spread because one bad round can make the extreme spread pretty...extreme.

But standard deviation accounts for that.

In this context, it's simply a way to define how consistent in velocity your rounds are.

(In statistics, using a distribution that is normally distributed--a normal curve--about 68% of all data points will be within 1 standard deviation of the mean. About 95 percent of all data points will be within 2 standard deviations of the mean. But this is for a population of data points whose distribution approximates a normal curve--or for the standard error of the mean, but you don't want to go there).

I focus on SD as a basis for consistency. It's more stable as a measure, takes into account the extreme shots if any but not overly so.

I've read that under one standard deviation for rifle rounds indicates pretty decent consistency (such as a SD of 28 when your average velocity is 3100). I've generally been able to achieve that w/ .223.

I can't achieve that with handgun brass, in part because I'm using many-fired brass of varying headstamps. I consider 2 standard deviations to be reasonable given the handgun ammo I'm shooting.

Finally, the number of data points matters. I generally don't pay any attention to SD unless I have at least 10 shots in a string. It will give me a good idea, but not perfect idea, of how that load tends to be in terms of consistency.

I'm testing some new bullets tomorrow. I'll try to shoot them through my chrono (assuming the range isn't too busy). But I only have 5 or so of each load, which I'm primarily testing to ensure I am at reasonable pressures/velocities. Once I'm comfortable that I won't be out of bounds with these, then I'll do a more comprehensive set of loads for accuracy testing.

I won't care about the SD much except that I want those shots to group around the mean in a reasonable manner--say within a couple SDs max. But if I shoot 3 or 5 rounds, that's not a sample size that'll be reliable for me.

March 11, 2010, 01:02 AM	#12
mongoose33 Senior Member Join Date: June 23, 2009 Location: Wisconsin Posts: 228	Standard deviation is simply a way to quantify spread of a series of data points, like velocities. It's better than the extreme spread because one bad round can make the extreme spread pretty...extreme. But standard deviation accounts for that. In this context, it's simply a way to define how consistent in velocity your rounds are. (In statistics, using a distribution that is normally distributed--a normal curve--about 68% of all data points will be within 1 standard deviation of the mean. About 95 percent of all data points will be within 2 standard deviations of the mean. But this is for a population of data points whose distribution approximates a normal curve--or for the standard error of the mean, but you don't want to go there). I focus on SD as a basis for consistency. It's more stable as a measure, takes into account the extreme shots if any but not overly so. I've read that under one standard deviation for rifle rounds indicates pretty decent consistency (such as a SD of 28 when your average velocity is 3100). I've generally been able to achieve that w/ .223. I can't achieve that with handgun brass, in part because I'm using many-fired brass of varying headstamps. I consider 2 standard deviations to be reasonable given the handgun ammo I'm shooting. Finally, the number of data points matters. I generally don't pay any attention to SD unless I have at least 10 shots in a string. It will give me a good idea, but not perfect idea, of how that load tends to be in terms of consistency. I'm testing some new bullets tomorrow. I'll try to shoot them through my chrono (assuming the range isn't too busy). But I only have 5 or so of each load, which I'm primarily testing to ensure I am at reasonable pressures/velocities. Once I'm comfortable that I won't be out of bounds with these, then I'll do a more comprehensive set of loads for accuracy testing. I won't care about the SD much except that I want those shots to group around the mean in a reasonable manner--say within a couple SDs max. But if I shoot 3 or 5 rounds, that's not a sample size that'll be reliable for me. Last edited by mongoose33; March 11, 2010 at 06:39 AM.