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What is standard deviation?

moredesmoredes Member Posts: 53 ✭✭
edited December 2001 in Ask the Experts
What does this term mean in relation to rifle ballistics?
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Comments

  • oneshyoneshy Member Posts: 417 ✭
    edited November -1
    It is the difference in velocity one can expect from rd. to rd. It is derived from the average of a 10 rd. string. It is used to show the consistency of a particular brand of ammo or reload. The smaller the no. the more consistent the ammo.
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  • moredesmoredes Member Posts: 53 ✭✭
    edited November -1
    oneshy, thanks for answering. What's the difference between this definition and 'velocity spread'?
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  • robsgunsrobsguns Member Posts: 4,581 ✭✭
    edited November -1
    Average velocity= Total of all velocities divided by the number of shots, ex. 30,000 is the total of 10 shots fired at 3,000 fps. each; 30,000 divided by 10 equals 3,000 fps avg.Extreme Spread= high velocity-low velocity;ex. 5 shots fired, high velocity is the fastest shot fired, low velocity is the slowest shot fired, the fastest shot was 3000 fps, the slowest shot was 2995 fps. so the extreme spread is 3000-2995 for 5fps extreme spread.Average Deviation= Sum of Differences divided by 10, when using a 10 shot string.ex. Lets say you fired ten shots at the following velocitities, 2995, 2990, 2998, 2990, 2995, 2999, 3004, 3001, 2899, 2997. Your average velocity is the total of the 10 shot string divided by 10 which is 2987 fps. You now must take your average and figure out the difference between it and each shot. ex. the first shot was 2995 fps so subtract 2987 from 2995 and the difference is 8fps. do that for each shot and then add all the differences up together. Lets just say that number of all ten added together equals, say 64. Now take the 64 and divide it by the number of shots fired, 10, that number is 6.4, so your average deviation is 6.4 fps. Now to take it one step further; energy= average velocity x average velocity x weight of bullet in grains, divided by 450,240, this will give you the amount of energy for your bullet in foot pounds. ex. 2987 x 2987 x 150 gr. bullet divided by 450,240= 2972 ft. lbs. of energy, at the distance from the muzzle that you chronographed the round which is usually about 15 feet. If this has you confused let me know and I will try to elaborate better.
    SSgt Ryan E. Roberts, USMC
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  • moredesmoredes Member Posts: 53 ✭✭
    edited November -1
    robsguns, thanks very much for the explanation. I followed it; the way you explained it made it quite easy, actually. I'm not trying to be fussy here, but are average deviation and standard deviation the same thing? (I'm unsure because of statisticians' use of words like 'average', 'mean', and 'standard'). Thanks very much for all y'all's time.
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  • robsgunsrobsguns Member Posts: 4,581 ✭✭
    edited November -1
    As far as I know average and standard are one in the same, it just depends on whos doing the talking, average deviation comes from 'deviation' from the 'average velocity', takeing this into account I would suppose that some might use standard the same as they would use average to express themselves.
    SSgt Ryan E. Roberts, USMC
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  • ref44ref44 Member Posts: 251 ✭✭✭
    edited November -1
    S.D. is a statistical term which is a measure of how well a group of data are grouped. It is not as simple to calculate as indicated here. You have to consider the square root of the sum of the squares of all the deviations from the mean. Rather than trying to understand S.D., consider this:If you take a random and unbiased set of numbers and calculate the S.D., then the mean (average) +/- 1 S.D. will include 68% of the data. The mean +/- 2 S.D. will include 95% of the data. And the mean +/- 3 S.D. will include 99.7% of the data. You may have heard the expression "95% Confidence Limits". In a string of shots (20 or more), it is 95% certain that the next shot will fall within the mean +/- 2 S.D.
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  • robsgunsrobsguns Member Posts: 4,581 ✭✭
    edited November -1
    Geez, ref44 makes me feel stupid, I still dont know what he said, but I feel I should.
    SSgt Ryan E. Roberts, USMC
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  • v35v35 Member Posts: 12,710 ✭✭✭
    edited November -1
    Standard deviation is the square root of the average value of the squares of the deviations fron the arithmetic mean while average or mean deviation is the absolute variation of the individual deviations from the arithmetic average.They're different.
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  • robsgunsrobsguns Member Posts: 4,581 ✭✭
    edited November -1
    v35, thanks, that clears it up perfectly, now I have definitive proof that I am stupid from 2 individuals in one evening. Thank God I know for a fact that whatever standard deviation is, I dont need it to evaluate MY loads, and if I did need it, I guess I'd just skip that step. Math is not one of my strong points.
    SSgt Ryan E. Roberts, USMC
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  • Master7thSonMaster7thSon Member Posts: 229 ✭✭✭
    edited November -1
    Yeesh!
    this stuff could seriously hurt your brain
    I think if we stopped to ponder this for very long we'd probably quit reloading. LOL
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  • ndbillyndbilly Member Posts: 1,573 ✭✭✭✭✭
    edited November -1
    This must be the 10th time in as many years that I have tried to follow an explanation of Standard Deviation. Still couldn't explain it if my life depended on it!. I'll keep the information in the same compartment of my brain where I've stored all I ever heard about ballistic coefficient. That, too, remains a mystery.
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