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# ? on ES / SD

#8
06-01-2008, 04:49 PM
 Platinum Member Join Date: Feb 2004 Location: Pennsyltucky Posts: 2,623
so just what is the standard deviation? isn't it simply a number that is exactly half of the ES? i understand the average velocity can be something that's not quite in the middle but i've always thought SD to be half of the ES and i'm like someone else that said it's kind of a "BFD" type of thing.
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#9
06-01-2008, 06:12 PM
 Silver Member Join Date: Feb 2008 Posts: 121
Not always-but acceptable

[quote=goodgrouper;209587]On a 3 shot group, the standard deviation will always be half of the extreme spread.,,,,,,,,,,,,,,,,

That's Wrong, unless the "middle shot" velocity is exactly at the middle of the ES!

Peter
#10
06-01-2008, 06:41 PM
 Silver Member Join Date: Mar 2008 Location: Chatham, VA Posts: 436
Quote:
 Originally Posted by Mikecr Why would you want SD anyway? You have ES, and that's all that matters. If I put a chrono on the market today, it would not display SD or any other monkey math. Just the raw, brutal truth..
+1 on that.

Low ES is the tougher standard.
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#11
06-01-2008, 11:55 PM
 Platinum Member Join Date: Sep 2004 Location: on the rifle range in Utah Posts: 2,723
[QUOTE=P KUNDA;209709]
Quote:
 Originally Posted by goodgrouper On a 3 shot group, the standard deviation will always be half of the extreme spread.,,,,,,,,,,,,,,,, That's Wrong, unless the "middle shot" velocity is exactly at the middle of the ES! Peter

No it is not really wrong. Since most chronographs round up or round down the speed to the closest number, for our purposes, the es will be basically double the sd on a 3 shot group plus or minus one foot per second which is better than the accuracy level of any chronograph can measure. Remember, this thread was started by a fellow who was asking how he could figure SD since (I assume) his chronograph wouldn't measure it on a 3 shot group like the CED M2. By taking the ES and dividing it by two, it will usually be the SD within 1 up or down (sorry about that gross non-absolute all you math nuts).

I probably should have elaborated on my original post that it is not "always exactly" half but it is always going to be basically half for the poster's needs. I should have realized that the math junkies were going to break it down to the letter of the law and would not allow a generalization!

I am sitting here looking at 25 pages of load data from the last several months with about 10 load ladders of 3 shot groups per page. In all that info, I only found two loads that the ES divided by two was more than one foot per second off the calculated SD. They were both 4 fps off. So the suggestion that you can take the ES and divide it by two to get the SD is pretty sound if you don't like running the longhand math like me! And remember, this is only for a 3 shot group. If you shoot more than 3 shots, the SD will begin to show it's gravity or importance even more.
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Last edited by goodgrouper; 06-02-2008 at 01:17 AM..
#12
06-01-2008, 11:58 PM
 Platinum Member Join Date: Sep 2004 Location: on the rifle range in Utah Posts: 2,723
Quote:
 Originally Posted by davewilson so just what is the standard deviation? isn't it simply a number that is exactly half of the ES? i understand the average velocity can be something that's not quite in the middle but i've always thought SD to be half of the ES and i'm like someone else that said it's kind of a "BFD" type of thing.
For all intensive purposes relating to figuring a load's capability, the sd will "basically" be half the extreme spread on a three shot group unless your chrono reads in half feet per second and not many do.

And if someone says that ES is everything and SD means nothing, they don't fully comprehend the fact that the two are intertwined to the point that either both are meaningful or both are meaningless.

My only gripe with holding ES as the holy grail of data is this:

Say you fire ten shots and they read
3000, 3001,3000,3003,3002,3000,3001,3005,3002, and then the last shot had a little tighter neck tension and it went 3050 fps. The ES method would show this load to have a spread of 50 fps and unless you went back and looked at each individual shot speed, you would think this load stunk. But the SD would still show this load kicked some serious butt. That is why both ES and Sd are helpful to have and therefore why both are included in the circuitry of modern chronographs.
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Last edited by goodgrouper; 06-02-2008 at 12:50 AM..
#13
06-02-2008, 12:23 AM
 Silver Member Join Date: Mar 2007 Posts: 430
Quote:
 Originally Posted by davewilson So just what is the standard deviation?
2. For each shot, subtract the average velocity from that particular velocity, and square the difference.
3. Get the sum of the squared figures.
4. Divide the sum by the number of shots. This number is the variance.
5. Get the square root of the variance. This number is the standard deviation.

The standard deviation is a measure of error that is useful when working with statistical inference based on the Gaussian distribution (bell curve). Other measures of error, like the extreme spread, may be more practical figures, depending on the application.

Quote:
 Isn't it simply a number that is exactly half of the ES?
Nope.
.
#14
06-02-2008, 01:33 AM
 Silver Member Join Date: Dec 2007 Location: bakersfield ca. Posts: 126
Quote:
GG, Thanks for the explanation and I now understand how you can have a high extreme spread and still have a low SD.

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