S&sprecisionTx
Silver $$ Contributor
Which one do you think's more important a low SD or a low ES
ES or SD
- Thread starter S&sprecisionTx
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The two measures are closely related, so are two sides of the same coin. The problem with simple ES is that a single outlier has a major effect on the value even if all other readings in the string are close to each other, or in an extreme example, have the same value. However, if the ES is small, the SD also has to be small, so if you get a single figure ES value for a shot-string, forget the SD other than for bragging purposes.
SD is a statistical tool based on variances from the arithmetic mean of all values in the string and tells you the likelihood of shots being within X or Y % of that mean. Statistically, it is more useful than plain ES which is why most people say a small SD value is the more valuable and representative measure.
The caveat with both measures is that their usefulness is highly dependant on the sample size. Frankly, SD values based on say 5 shots are of very limited value, and only start to be reliable with 10 or more. Even at a sample size of 10, most engineers, scientists and statisticians say that sample is too small and you need a lot more to obtain a proper picture. Then too, nobody gives you a match medal for having the smallest SD value of any competitor's ammunition - it's the effect on the target that counts. The statisticians tell us that ES/SDs show up as large 'elevations' on long-range targets. I've often questioned some of the figures bandied about as they don't always appear to correlate with hit patterns, and in any event, spreads at the muzzle sometimes change in flight through physical bullet variations affecting their drag, not to mention ambient environmental variations over long flights.
I'm assuming you're referring to MV readings. ES/SD applies to other things we do in shooting that involve measurement. Group sizes based on edge to edge / centre to centre readings will normally show a range of values if you shoot 10 groups in a string (allowing barrel cooling to avoid external influences) and statistically need to have their ES/SD calculated. Bryan Litz goes into this in considerable depth in his book Modern Advancements in Long Range Shooting Vol 3 and illustrates the large range of group sizes obtained experimentally by a team shooting 50 by 5-round groups with a good .22LR match rifle and quality ammunition. That's why military or other ammunition quality testers don't use the C to C method, but instead identify the central group point and measure the range of distances from it to each individual shot, thus reducing the effects of individual fliers on the statistics. (Not much use to BR competitors though!) So, in almost any precision shooting discipline, ES in this respect is everything and nobody other than manufacturers or military testers talk SD.
SD is a statistical tool based on variances from the arithmetic mean of all values in the string and tells you the likelihood of shots being within X or Y % of that mean. Statistically, it is more useful than plain ES which is why most people say a small SD value is the more valuable and representative measure.
The caveat with both measures is that their usefulness is highly dependant on the sample size. Frankly, SD values based on say 5 shots are of very limited value, and only start to be reliable with 10 or more. Even at a sample size of 10, most engineers, scientists and statisticians say that sample is too small and you need a lot more to obtain a proper picture. Then too, nobody gives you a match medal for having the smallest SD value of any competitor's ammunition - it's the effect on the target that counts. The statisticians tell us that ES/SDs show up as large 'elevations' on long-range targets. I've often questioned some of the figures bandied about as they don't always appear to correlate with hit patterns, and in any event, spreads at the muzzle sometimes change in flight through physical bullet variations affecting their drag, not to mention ambient environmental variations over long flights.
I'm assuming you're referring to MV readings. ES/SD applies to other things we do in shooting that involve measurement. Group sizes based on edge to edge / centre to centre readings will normally show a range of values if you shoot 10 groups in a string (allowing barrel cooling to avoid external influences) and statistically need to have their ES/SD calculated. Bryan Litz goes into this in considerable depth in his book Modern Advancements in Long Range Shooting Vol 3 and illustrates the large range of group sizes obtained experimentally by a team shooting 50 by 5-round groups with a good .22LR match rifle and quality ammunition. That's why military or other ammunition quality testers don't use the C to C method, but instead identify the central group point and measure the range of distances from it to each individual shot, thus reducing the effects of individual fliers on the statistics. (Not much use to BR competitors though!) So, in almost any precision shooting discipline, ES in this respect is everything and nobody other than manufacturers or military testers talk SD.
BoydAllen
Gold $$ Contributor
On the outliers, the target sees them.
IMO the reason that we hear so much about SD is general lack of knowledge about the need for a reasonable sample size. Chronographs will calculate SD for any number of shots, no matter how ridiculously small the sample size is and I think that people take more comfort in the smaller number.
When I read of someone giving the SD of a three or five shot group, I smile, knowing that if the writer knew what he was doing he would stick with ES, which is of course simply the difference between the lowest and highest velocities in a group.
Where SD might be valid would be if you are confident that you have the right load and you chronograph all shots of a practice session shot with that load as a single string. If your total was at least twenty, better yet thirty shots, then you might legitimately have some confidence in the SD calculated from all of those shots.
IMO the reason that we hear so much about SD is general lack of knowledge about the need for a reasonable sample size. Chronographs will calculate SD for any number of shots, no matter how ridiculously small the sample size is and I think that people take more comfort in the smaller number.
When I read of someone giving the SD of a three or five shot group, I smile, knowing that if the writer knew what he was doing he would stick with ES, which is of course simply the difference between the lowest and highest velocities in a group.
Where SD might be valid would be if you are confident that you have the right load and you chronograph all shots of a practice session shot with that load as a single string. If your total was at least twenty, better yet thirty shots, then you might legitimately have some confidence in the SD calculated from all of those shots.
This is how I boil it down for me in terms of velocities: SD's tell me how well I'm loading my cartridges and ES's mean little in that a single outlier can distort what's going on (though if I have a particular case that keeps generating a significant outlier, I'll be sure to pull it and not use it again). Even on paper, ES's aren't as important to me as the Mean Radius of a group is.
dcali
Bullet Maker
Shoot enough, and you'll discover that they're in essence the same thing. Practically, SD is going to be a more reliable measure because it takes all of your shots fully into account. ES ignores a lot of information.
For example, shoot 25 or 30 shots, and calculate the SD and ES for the whole group. And then do exactly the same thing again. I'd bet good money that the SDs are the same (or very close) and the ESs are different. The ammo is the same, so which is the better measure? The SD.
If you're only going to shoot 3 or 4 shots, don't bother. It's not going to tell you anything useful.
For example, shoot 25 or 30 shots, and calculate the SD and ES for the whole group. And then do exactly the same thing again. I'd bet good money that the SDs are the same (or very close) and the ESs are different. The ammo is the same, so which is the better measure? The SD.
If you're only going to shoot 3 or 4 shots, don't bother. It's not going to tell you anything useful.
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BoydAllen
Gold $$ Contributor
Do you know when an "outlier" will occur? Can you identify one by looking at the cartridge? If you have a batch loaded and are taking them to a match can you tell the person scoring to ignore a shot because it is an outlier? The target shows all shots, so you need to know what the total range of velocities is because it only takes one shot to spoil a group or score.
In my experience SD (which i use) is more accurate at predicting an outlier due to ES (vertical dispersion as a result of velocity)if the sample string is sufficient. I never use a string shorter than 30 rounds and most often use 50 to determine base SD. There is another good thread on this very subject here
read post 7 by Bryan Litz.
ES and SD
I am new to competitive shooting and reloading. I reload for my varmint rigs,223, 22-250). I am now reloading for a F-class 6.5/284, and read alot about ES and ED. How do you calculate ES and SD. I under stand extreme spread, the difference between the high and low velocity, but how do you...
forum.accurateshooter.com
read post 7 by Bryan Litz.
Hi-NV Shooter
Gold $$ Contributor
dcali
Bullet Maker
If a shot is truly and outlier, knowing the ES isn't going to help (my definition of an outlier is a shot that does not follow the statisitcs for whatever reason, not one that is on the edge of the curve).
ES isn't worthless, but it's my opinion that using a well founded SD to determine the ES you can expect (say, 3 SDs in either direction) is better than actually trying to measure the ES because SD contains a lot more information.
SD contains the information of every shot, where ES is based on two shots, and the knowledge that all the rest are in between. In practice, if you shoot a large sample, ES and the ES predicted by SD tend to be pretty much the same. If you have a true outlier, then all bets are off. No amount of statistical knowledge will save you.
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The science of ballistics and statistics doesn’t differentiate between RF and CF!
You may get vastly different results between the two, but they both reveal what’s truly happening when we test and analyze the data gathered.
As regards ES/SD, SD is the superior statistical metric if you’re quantifying a better load in CF or a better lot of RF ammo.
I would only use ES if I were attempting to measure the vertical dispersion of either on target, but even in that case, using the metric “Slope” is a better choice.
Landy
You may get vastly different results between the two, but they both reveal what’s truly happening when we test and analyze the data gathered.
As regards ES/SD, SD is the superior statistical metric if you’re quantifying a better load in CF or a better lot of RF ammo.
I would only use ES if I were attempting to measure the vertical dispersion of either on target, but even in that case, using the metric “Slope” is a better choice.
Landy
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