bob
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I'm too old to be nice but never too old to learn!
Posts: 1,457 
Location: Northern California
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Post by bob on Mar 25, 2015 19:42:15 GMT -5
In the realm of shooting what value is there to knowing the standard deviation? I know what it is and how to calculate it but frankly I see no value. It has no apparent correlation to group size and you can't determine extreme spread as that is part of the calculation. Actual average velocity and extreme spread have more value to me, I think gun writers just uses SD to sound intellectual.  |
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gws
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Posts: 969
Location: NW New Mexico
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Post by gws on Mar 26, 2015 10:49:08 GMT -5
I'll be honest with you. I've known what it is and I have calculated it......but its been a while. These days I have to re-cram to get back knowledge I had yesterday, only to forget it again tomorrow. Ok, I exaggerate......maybe just a little. For what I do in the field repeatable group size has been ample data for me. But you know I now own a new chonograph....no, I haven't made it in the field to try it out. Been waiting for a nice sunny spring Saturday minus the usual cold north wind from Colorado.....hasn't happened yet. I need no cold distractions to learn something new. Anyway, I know such data is produced by the thing, so I went to my favorite mentoring forum to try to get re-educated. I found the link below, and I even understand part of it. Maybe you can soak up some light. thefiringline.com/forums/showthread.php?t=552450&highlight=standard+deviationBTW, (for anyone happening along) the only data a chronograph can give relates to velocity. So I'm assuming that's what we are talking about here, not group size, but the extreme spread (ES), and standard deviation (SD), in the velocity data. Statistically you can get ES and SD statistics off the physical target as well, but we usually refer to group size and flyers for that....and MOA and SubMOA...etc. Nuther subject. However, Chrono. ES and SD data certainly effects the target.......as do wind, temperature, atmospheric pressure, and most of all the human factor. Seems to me, that one thing about the Chrono. stats is that for the most part those other factors are minimized, since you are usually shooting from a bench through sensors a whole 10' away. That's useful right? Anyway read the thread at firing line and tell me if you gain anything ....... and share of course. |
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bob
Member
I'm too old to be nice but never too old to learn!
Posts: 1,457
Location: Northern California
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Post by bob on Mar 26, 2015 14:32:03 GMT -5
Greg: Thanks for that link, there are two very good explanations of SD one basic and one technical. This SD dilemma has be with me for a long time and just recently reared it head again. I was looking at a review of some cartridge/rifle/load and the group size did not correlate with the SD. Some SDs were close and the groups were good and some SDs were good and the group size was large. The author gave the average velocity and the SD and the group size,I feel the Extreme Spread would have bee better information. I will read those links later as I'm on lunch break now and have a few men awaiting instruction. (have to be retrained after a break ya know)
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Post by krwada on Mar 26, 2015 18:43:44 GMT -5
You can get very low SD on the chrono and still get terrible groups. You can also get very low SD AND ES on the chrono and still get very bad groups.
All the SD and ES information tells you is what the muzzle velocity is doing.
I can safely say that if you get terrible SD and ES, then you will most likely will get terrible groups too.
At the end of the day ... accuracy and precision is dependent on a bunch of variables. Muzzle velocity and especially consistent muzzle velocity is just one of the many variables.
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Post by hacker54 on Mar 27, 2015 22:53:34 GMT -5
This is nice info to learn and know but since I no longer have a chrono I go by the end results. How I do on target and what distance I'm shooting at. I would guess that if my groups on target are consistent then I think it be a safe bet that the velocities are consistent also. I would also guess that the ES in this scenario would be small. Would this be the case? As I stated things that are nice to know but I go by on target results. Back when I was in training to become an EMT I had to know how blood went through the body as it started in one chamber of the heart and yes that question was on my state written exam. I could careless if pumped Purple Puppy Pi.... All I cared about was what type of bleeding and how to stop it.
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My daughter came from Missouri, my son and grand son, my wife and I had a nice dinner of grilled "porter house" pork chops, corn on the cobb, my son, G'son and I share 10/5. G'son is 4 minutes early though @ 10/4.
Pooh I hope you find to right house . 
