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Can someone please , in simple terms, explain to me the formula for calculating this? I have an accurate load for my 223. Five shots over the Chrony yielded the following 1. 3024 2. 3131 3. 3085 4. 3088 5. 3066 Thanks in advance rob "the older I get, the better I was" | ||
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Put the figures into an Excel spreadsheet and tell it to calculate the standard deviation. ________________________ "Every country has the government it deserves." - Joseph de Maistre | |||
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Winchester 69 is totally right. The easiest way is to have a calculator that will perform the calculation for you. Excell can do this. Your 5 speeds yield a standard deviation of 38.77757084 | |||
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How does the load shoot? That is really what is important. I have had some very accurate loads shoot into double SDs. Your shots #1 & #2 have the largest swings. That will certainly show up @ longer range, but again, shoot it @ longe range & see how it shoots. LIFE IS NOT A SPECTATOR'S SPORT! | |||
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How do you do it with excell? | |||
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Johan Go to the cell and type =Stddev this should bring up the fromula options pick the stddev formula and include the range of values by dragging the cursor over the range. Dis al wat jy moet doen ! Regards "Man is a predator or at least those of us that kill and eat our own meat are. The rest are scavengers, eating what others kill for them." Hugh Randall DRSS, BASA 470 Krieghoff, 45-70 inserts, 12 ga paradox, 20 ga DR Simson/Schimmel, 12 ga DR O/U Famars, 12 ga DR SXS Greener | |||
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Thanks for the advice. The load shhoots to an average of .88 at 100 yds. Smallest group went into .323. Just curious about SD as it keeps appearing in articles etc and as I'm a believer that the only stupid question is the one you don't ask, I thought I would learn something that may or may not be of real world use. Thankyou to all rob "the older I get, the better I was" | |||
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Long hand calculation is easy but time consuming. First you need to find the MEAN. Add up your figures then divide by the number of figures. 3078 in this example. Then subtract each of your numbers from the mean. This will give you the deviation from the mean for each number. 3078 - 3024 = 54 3078 - 3131 =-53 3078 - 3085 =-7 3078 - 3088 = -10 3078 - 3066 = 12 Now you square each of the above deviations. 54 x 54= 2916 -53 x-53= 2809 -7 x -7= 49 -10 x-10= 100 12 x 12= 144 Now add the squares of the deviations 2916 +2809 +49 +100 +144 =6018 Now divide by (n-1) where n is the number of events your are tracking. In this example n is five shots so (5-1)=4 So divide 6018 by 4. 6018/4 = 1504.5 Lastly you get the square root of this last number. This is where I need a calculator. the sq rt of 1504.5 = 38.79 This is your standard deviation. muck | |||
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Thanks Muck, After reading all that Excell is starting to look mighty attractive! rob "the older I get, the better I was" | |||
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Except standard deviations don't mean anything in this example. First, the sample size typically has to be greater than 30 Second, all standard deviation tells you is that if you repeat your process over and over say 1000 times that: Nominally 68% of all the values will fall with in the range of +/- 1 std dev 95% +/- 2 std devs and 99.7% +/- 3 std devs So lets say you actually fired another 30 shots and got very similar distribution of velocities and the Std Dev calculated to be the same. You would now know that 99.7% of the time your velocity will be between 2986 and 3194. So unless you are using a sample size of 35-40 shots with an average velocity of say 3000 and geting a std dev of about 7 haven't accomplished much. Mike Never under estimate the internet community's ability to reply to your post with their personal rant about their tangentially related, single occurrence issue.  What I have learned on AR, since 2001: 1. The proper answer to: Where is the best place in town to get a steak dinner? is…You should go to Mel's Diner and get the fried chicken. 2. Big game animals can tell the difference between .015 of an inch in diameter, 15 grains of bullet weight, and 150 fps. 3. There is a difference in the performance of two identical projectiles launched at the same velocity if they came from different cartridges. 4. While a double rifle is the perfect DGR, every 375HH bolt gun needs to be modified to carry at least 5 down. 5. While a floor plate and detachable box magazine both use a mechanical latch, only the floor plate latch is reliable. Disregard the fact that every modern military rifle uses a detachable box magazine. 6. The Remington 700 is unreliable regardless of the fact it is the basis of the USMC M40 sniper rifle for 40+ years with no changes to the receiver or extractor and is the choice of more military and law enforcement sniper units than any other rifle. 7. PF actions are not suitable for a DGR and it is irrelevant that the M1, M14, M16, & AK47 which were designed for hunting men that can shoot back are all PF actions. 8. 95 deg F in Africa is different than 95 deg F in TX or CA and that is why you must worry about ammunition temperature in Africa (even though most safaris take place in winter) but not in TX or in CA. 9. The size of a ding in a gun's finish doesn't matter, what matters is whether it’s a safe ding or not. 10. 1 in a row is a trend, 2 in a row is statistically significant, and 3 in a row is an irrefutable fact. 11. Never buy a WSM or RCM cartridge for a safari rifle or your go to rifle in the USA because if they lose your ammo you can't find replacement ammo but don't worry 280 Rem, 338-06, 35 Whelen, and all Weatherby cartridges abound in Africa and back country stores. 12. A well hit animal can run 75 yds. in the open and suddenly drop with no initial blood trail, but the one I shot from 200 yds. away that ran 10 yds. and disappeared into a thicket and was not found was lost because the bullet penciled thru. I am 100% certain of this even though I have no physical evidence. 13. A 300 Win Mag is a 500 yard elk cartridge but a 308 Win is not a 300 yard elk cartridge even though the same bullet is travelling at the same velocity at those respective distances. | |||
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http://www.physics.csbsju.edu/...stats_NROW_form.html Works great. Just tell it how many samples you have, put 'em in the boxes and click. Bingo! Your values are right there... | |||
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Rob The smaller the SD the better the capability, repeatability ( CpK ) These manufacturing terms, can be applied to reloading. But as stated above they are more applicable to mass production and lean manufacturing. So unless you are planning on making a living from reloading I would not worry too much. I worry more about ES (Extreme spread) in my loads. Enjoy. "Man is a predator or at least those of us that kill and eat our own meat are. The rest are scavengers, eating what others kill for them." Hugh Randall DRSS, BASA 470 Krieghoff, 45-70 inserts, 12 ga paradox, 20 ga DR Simson/Schimmel, 12 ga DR O/U Famars, 12 ga DR SXS Greener | |||
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Hmmmm... It looks like I've been calculating the "Average Absolute Deviation from Median" all these years! | |||
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Wow! Three threads away, there's a discussion of 'accuracy 100 years ago' and a story about an old timer who did some phenomenal shooting. Now we need a computer with specific software to tell us if our ammo is up to snuff. I think I'll just keep on measuring my groups with my little pocket tape measure - if they please me, fine...if they don't, I'll tinker some more. Life is getting too damn complicated. | |||
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So why then, are there constant references to SD? I've found, from my limited shooting experience, that some of the loads I've shot have printed tiny groups , yet there was a significant variation in shot to shot veocities. Other loads have shown a shot to shot variation of just a few feet per second, yet have grouped all over the target. I think I'm going to take a headache tablet and lie down for a while! rob "the older I get, the better I was" | |||
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Aggravating isn't it? From having wasted a whole lot of time reading Hack writers in Gun Rags, it is generally because it is easy to copy off the info(not Data) spewing from the chronograph. And few if any of them have a real handle on it. Mike's explaination is pretty darn good. It basically tells you how consistant the Velocity is from a Norm(the Average). And Muck did a fine job with the math, so anyone can follow the process. Up close, Standard Deviation doesn't really amount to a whole lot. But w-a-y out yonder, it can have an influence on the Group size. But..., there are always examples where a large S.D. still shot tiny Groups. Generally these can be attributed to Random Group Dispersion of those individual shots. So, folks quote all the S.D.s, B.C.s, Velocities, Trajectories and anything else they desire to confuse the issue with, but it always gets back to looking at the actual Groups made at the distance you desire to take shots. Shoot the Load enough at that distance and the Groups will indicate if it is a good Load/Trigger Yanker combination, or something that needs to be improved on. And no stinkin' Chronograph is needed. Best of luck to you. | |||
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It's the chrono mfrs' fault - it gets calculated.
Long-distance shooting will show a taller vertical spread if the velocity varies. ________________________ "Every country has the government it deserves." - Joseph de Maistre | |||
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I'm with Hot Core on this one..... Standard deviation isn't exactly a number I'm very intersested in.....unless I take up 600+ yard shooting some day.....and it's not too likely. /////////////////////////////////////////////////////////////////////////// "Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy, its inherent virtue is the equal sharing of misery." Winston Churchill | |||
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I flunked Stats in school-- flunked it hard. But from what I know, SD is a measure of how "same as" or "different from" a set of values are from each other. If the SD is small, the values are all pretty close to each other. In reloading ammo, that's what we want-- that consistency. If the SD is large, that means you're values are all over the highway-- inconsistent and not much good for trying to launch the bullets same-same from one to the next. For all you guys who know Stats, am I right, more or less? | |||
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Rob, I use an excel spreadsheet to keep my reloading notes - It helps keep every thing together. I keep a sheet on each rifle as follows:
So...Contrary to the statisticians, who will never have a large enough sample size and the nay sayers that don't use $hit, I find I'm see patterns to the stats the help me produce CONSISTENT and accurate reloads! ________ Ray | |||
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Gee guys, Many thanks for all the input! I can see where these variation in velocity can have an effect over longer ranges, but out to where I plan to shoot it won't matter that much. I'm also impressed that the number of posts got to 20 without any name calling or references to family trees and upbringings coming into play. Thanks to all who contributed. I'm shooting 55 gn Sierras and am trying to sort out which powder will work best. So far Benchmark has produced the goods, buy H 4895 showed some promise today, but I need to play morew with this powder. rob "the older I get, the better I was" | |||
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It got close... | |||
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Sample size matters if you want "statistically significant" results. A very different concern is whether you have results that are "practically" significant. For some people that "practical" difference might be different than for other people. If you are shooting a 1000 yard match, small variations within a bunch of loads might matter. I, for one, very seriously doubt that any animal on the planet will know the difference between a few feet per second even if the statistical descriptions are "stat sig" or if the std. deviation is recognized as "big". For me then, practical significance falls to near zero unless we are talking about complete squib loads sneaking into the mix. | |||
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Same-E-Same with my 223Rem. I use Benchmark for the old, "Who buys the BBQ Loads?" H4895, WW-748 and BL-C2 are also right accurate in mine. A cartridge that performs well with many different Powders is an excellent design. and the 223Rem is one of them. Best of luck to you. | |||
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Easy Calculation is the website I use. My dad told me once that if you're gonna kill a rattler with a chainsaw, use the top of the bar. | |||
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Tumbleweed, et al, you are right. I think we tend to over think this thing. We measure meaningless "stuff" cause the 'puter will do it for us. If it's a good load, who cares what the SD is? Or many of these other things that folks are constantly measureing. Getting a small SD is NOT a guarentee of accuracy. Aim for the exit hole | |||
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Back in the dark ages before PCs, I had a job in QC and spent many, many hours calculating SD, ES, Coefficient of Variation and much other esoteric data on sample sizes numbering in the hundreds and even thousands. Had to have a hardcopy printout of all calcs since it was nuclear work; I did it all on a desk-top calculator, 'longhand' as it were, with final printout tapes measured in feet, not inches. I agree with everything said above, FWIW. Using the SD function is VERY MUCH like the old 'string measure' used by shooters of the 19th and early 20th centuries for measuring a rifle's accuracy. The result isn't exactly the same of course but the principle is the same, it's a measurement of how close any given succeeding shot is likely to fall in relation to the norm of the others. Maybe useful for choosing an artillery-shell supplier but not for our purposes. Regards, Joe __________________________ You can lead a human to logic but you can't make him think. NRA Life since 1976. God bless America! | |||
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Thanks for the link Doctor D. muck | |||
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wasbeeman has the short and sweet of this subjact. muck | |||
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Have to agree! A sample size this small has little statistical confidence. Not that it doesn't have any meaning. Just not very reliable. muck | |||
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