Decent SD but High ES

[moe africa]

Hi
I have found a load that is giving me an SD of 8 and an ES 29 fps over 10 shots
It's grouping well yet I don't know why the ES is so disproportionate
Attached is a 9 shot group at 100m

 

[Fuj]

Did you have one or two "extremely" low or high velocities
in the mix ??

 

[moe africa]

Did you have one or two "extremely" low or high velocities
in the mix ??

Yes I had one outlier that was about 20fps less than the mean
It was statistically an outlier
But I don't know if I should rely on statistics to interpret the data or just take it as it is

 

[big5ifty]

Run that target out to 300, and if the groups are still that good, you have a good load.

Statistically, the few shots we use to determine ES can be misleading, because there is a probability you may get the highest and lowest values in the same group, or just a few from the middle of the distribution.

Be aware of ES and SD, but concentrate more on what the target shows, ideally with no wind at 300.

 

[moe africa]

Run that target out to 300, and if the groups are still that good, you have a good load.

Statistically, the few shots we use to determine ES can be misleading, because there is a probability you may get the highest and lowest values in the same group, or just a few from the middle of the distribution.

Be aware of ES and SD, but concentrate more on what the target shows, ideally with no wind at 300.

I took 3 shots at 300 meters and it wasn't amazing conditions were not very great though

 

[big5ifty]

If you extrapolate the group size from 100 and compare to 300, my eyes tell me your 300 group is smaller than your 100 group.

You have an excellent load, and clearly you can shoot well enough for a good group.

If the bench is not immobile, shoot from prone off a stiff rear bag, and on a day with no wind, you may have a quarter MOA thing going on there.

I would not be concerned with the ES with results like that.

 

[moe africa]

If you extrapolate the group size from 100 and compare to 300, my eyes tell me your 300 group is smaller than your 100 group.

You have an excellent load, and clearly you can shoot well enough for a good group.

If the bench is not immobile, shoot from prone off a stiff rear bag, and on a day with no wind, you may have a quarter MOA thing going on there.

I would not be concerned with the ES with results like that.

This was shot off sand bags

 

[Doom]

On what basis did you call that shot an outlier? Statistically a 29 is not out of line with an 8 SD. While then shots is not a great sampling it is possible that these numbers are consistent.

 

[moe africa]

On what basis did you call that shot an outlier? Statistically a 29 is not out of line with an 8 SD. While then shots is not a great sampling it is possible that these numbers are consistent.

Mean - 1.5 x IQR

 

[moe africa]

If you extrapolate the group size from 100 and compare to 300, my eyes tell me your 300 group is smaller than your 100 group.

You have an excellent load, and clearly you can shoot well enough for a good group.

If the bench is not immobile, shoot from prone off a stiff rear bag, and on a day with no wind, you may have a quarter MOA thing going on there.

I would not be concerned with the ES with results like that.

I think it's more or less consistent if you see the mean radius which isn't dependent on shot count

 

[lotech]

ES and SD are indicators of consistency, not accuracy. Lots of factors contribute to making an accurate load. If your load is shooting well, don't worry about the ER and SD numbers. Verify your load is accurate by shooting a sufficient number of groups at whatever distances you normally shoot. If you want very small ER and SD numbers, shoot some compressed loads of slow burning powders. These loads might be accurate, but there's just as good a chance they won't be.

 

[284winner]

I find that the ES isn't noticed at closer ranges but rather out beyond 300.

 

[Doom]

Mean - 1.5 x IQR

I don't think a value that is 20 off the mean should be considered an outlier. especially with a SD of 8 unless you can identify a reason to exclude it, such as different brass, different primer or something else. Also the IQR formula is not based on mean but on median.

 

[moe africa]

I don't think a value that is 20 off the mean should be considered an outlier. especially with a SD of 8 unless you can identify a reason to exclude it, such as different brass, different primer or something else. Also the IQR formula is not based on mean but on median.

Typing mistake
My median and mean the were the same for this sample

 

[D-4297]

@lotech made a very telling comment , and many will agree with it . I'll take that statement one step farther , and hopefully , not get "flamed" for doing so . The "consistency" I feel he was referring to is in your Brass preparation . The more consistent your Brass is , as I have learned over years of doing this , the lower your SD becomes . The added benefit is that your "Groups" become smaller , too . Not to say that SD isn't a factor , but not quite the "Be All & End All" that some would like to think it is . I shot my first 200 - 20X on a day with 5 - 8 mph breeze , 90 degrees from the right . And the SD was 13.4 . So tell me how relevant , or important SD really is . I've since improved my Brass prep , and my current , usual SD runs around between 6 to 7.6 . Scores are consistently Mid to High 190's with a decent X count . Use what you have currently for your load , and focus on the Brass prep . I'll almost guarantee the SD will come down , and the scores will go up as the group gets tighter .

 

[CharlieNC]

You cannot truly have a decent SD and high ES, small sample sizes yield variable answers. For a suitable sample size ES= 6 SD as reality across the long term which you will ultimately experience. Anyway it's the target that matters.

 

[lotech]

@lotech made a very telling comment , and many will agree with it . I'll take that statement one step farther , and hopefully , not get "flamed" for doing so . The "consistency" I feel he was referring to is in your Brass preparation . The more consistent your Brass is , as I have learned over years of doing this , the lower your SD becomes . The added benefit is that your "Groups" become smaller , too . Not to say that SD isn't a factor , but not quite the "Be All & End All" that some would like to think it is . I shot my first 200 - 20X on a day with 5 - 8 mph breeze , 90 degrees from the right . And the SD was 13.4 . So tell me how relevant , or important SD really is . I've since improved my Brass prep , and my current , usual SD runs around between 6 to 7.6 . Scores are consistently Mid to High 190's with a decent X count . Use what you have currently for your load , and focus on the Brass prep . I'll almost guarantee the SD will come down , and the scores will go up as the group gets tighter .

As for brass preparation, I don't do anything extravagant like flash hole or primer pocket uniforming but will trim and turn case necks if necessary. However, with rifle brass in particular, I prefer to use new brass of the same batch. I don't anneal but rather throw brass in the salvage pile when it becomes worn. Group size is everything; I only pay attention to ES and SD numbers, neck concentricity, etc. when I use them as elimination factors if group sizes increase.

I'm not condemning flash hole and primer pocket uniforming, annealing, etc. These procedures probably have some degree of usefulness especially at very long range. I seldom shoot past 400 yards.

 

[RegionRat]

If the OP's SD were really 8, then you may end up with an ES of 6x8=48 for +/-3 sigma worth of the distribution. In other words, as the sample size of a normal distribution gets closer to the truth, the ES can grow to six times the SD value.

That said, with less than 30 samples of a well behaved distribution, you may not know much till you test more.

The advice to pivot off the target and not the chronograph is good. If the target is telling us the load is worth a closer look, then the velocity stats will follow. For the load to be good at distance, the velocity stats will end up being at least decent, which vice versa may not be true in that we have all seen great velocity stats that don't group.

 

[Doom]

I started this before RegionRat's post but I'll post it anyway as it elaborates a little more on the SD. It's import to remember that the SD that is measured for 10 shots is probably not the SD of a larger number of shots. Looking at the 95% confidence interval the likely true standard deviationtion is between 5.5 and 14.6. If more shots were fired its likely that the SD would increase and also the ES. Not much should be read into the ES numbers in general as it only reflects the results of 2 shots out of the test sample. As others have noted SD is a good indication of how consistent your reloading process is.

 

[MikeMcCasland]

Having high SDs doesn't really tell you much, because so many variables can contribute to it. High SDs aren't even a solid indicator that you're doing something wrong. I've seen barrels, and certain lots of bullets put up trash numbers. Certain bullet/caliber combos are also just inherently more impressive on the chronograph than others.

I challenge anyone to put up consistently sub-10FPS SDs with a 90gr/223 combo. Meanwhile I don't know if it's possible to get above a ~7FPS SD out of certain 308 combos.

Having consistently low SDs/ES on the other hand is a decent indicator that you're doing something right on the bench.

As it pertains to load development for a given distance, I think everyone prefers low numbers, but I've shot some really good scores with some loads that look pretty crappy on a chronograph.