idle curiosity: why SD instead of %RSD?


April 14, 2007, 09:00 AM
When discussing chronograph result, standard deviation is often reported as a measure of consistency for the load.

It seems to me that it would be easier to compare loads with different velocities using %RSD instead (percent relative standard deviation, or (SD / mean) * 100%). Using %RSD would make the task of selecting a single powder for use with multiple calibers or bullet weights easier.

Is there a reason that SD is more commonly used?

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April 14, 2007, 11:36 AM
Trying to relate this to issues I deal with professionally, I have to say "I don't have a clue." When I deal with this type of issue at work, I commonly calculate %RSD. The origins of the whole topic of probability always dealt with data that was mathematically simplified and normalized to a maximum value of one, the way mathematicians usually address theoretical topics. But then, the subject started out as being the domain of mathematicians, not practicalities didn't matter to them [showing my prejudicies again].

When I see SD values in shooting magazines, I usually mentally divide them by their mean to get a better relative picture of how tight the data grouped. It is simpler to just provide the SD, but %RSD is far more useful to me.

Mal H
April 14, 2007, 02:28 PM
I'll take a swag at why SD is used more often (or almost exclusively) in ammo quality determination.

To me, it is important that the SD be as small as possible. That is a direct indicator that all the steps involved in producing a round are getting closer to exactness, i.e., weights are almost exactly the same, cases have the same volume, bullets are seated equally, etc.

The PRSD is a good indicator of how far from the average a value might be. For example, if you are getting an average of 2500 fps from a group of 5 measured firings and the PRSD is 5%, then the predicted velocity of each round is 2500 +- 5%. I would have to say - so what? I already know what the average velocity is. The next round fired might be that average +- a percentage. Is that figure useful to me - no. I have no use for guessing what the velocity of any round will be nor do I need to know how far from the average each round in a group is.

Just like the SD, you want the PRSD to be as small as possible, but why do the extra math?

So what I'm saying is the SD is as good an indicator of quality as any other figure. Quality is what we're after when using SD. Velocity is, of course, very important to determine, but, to me, the range is not necessary to know, only the average.

April 14, 2007, 03:50 PM
because a SD of 10 fps with a mean velocity of 500 fps is huge, but a SD of 5 fps, with a mean velocity of 4000fps is awesome.

Jim Watson
April 14, 2007, 04:08 PM
My local PhD got interested in shooting and handloading, bought a chronograph and studied the issue. He favored relative standard deviation.

It is easier to get single digit SD with black powder in a .45-70 than with nitro in a .308, but sub 1% RSD is comparable.

The Bushmaster
April 14, 2007, 06:20 PM
I'm more interested in extreme spread (ES) then anything. If this number is small the rest of them will be also...:)

Chuck Dye
April 14, 2007, 06:49 PM
I like Mal H's take but suspect that the REAL reason is that there is a button on any decent calculator and some chronographs for Standard Deviation (σ), and not for %RSD.

April 14, 2007, 07:16 PM
BM: That is true for the most part except that you might have one or two rounds that are departures from the norm (depending on the number of rounds you use for a string) and inflate your extreme spread. After many years of doing this, you probably have a ratio built into your head that allows you to approximate an SD.

Myself, I do a lot of pressure calculations (hydraulic and sound) and after a period of time, some of the established values start imprinting themselves on your brain.

NavArch: I'm extrapolating an occupation from your handle here, but is it possible that when you calculate %RSD you are working with established values of materials, and other known physical values? Take heart, most of the meaningful research in chronograph technology over the past three decades or so has been done by Dr. Ken Oehler, and I believe his doctorate is in mathematics, if not Physics. Plus, he's a Texan!:D

If you have a need to know average deviation as you go along, there are chronographs that will provide that for you. The chronograph has to set the mean and then compare values to it.

I think Mal H makes a good point, or maybe what I'm taking from it. Say you're shooting groups while chronographing. Personally, I don't want the values during a string of fire unless something departs from the norm, otherwise, an apparently odd value may affect the norm itself as far as concentration and group size.

mek42: that notion would be pretty handy wouldn't it? Unfortunately, many of the powder companies just as soon you not know the SD characteristics of some powders, it might effect sales. Ramshot is the last company to have done this and if you see me rambling on about some of their powders, that's part of the reason why.;)

The Bushmaster
April 15, 2007, 08:46 AM
CZ57...I use a Pact mod 1 and a 5 round string for pistol and 3 round string for rifle. Anything under 50 fps ES is what I call a good string...:)

April 15, 2007, 06:59 PM
TBM: No problem, it was made in Texas too!:D I happen to know that it's a very good chrono . . . and if yours is as old as mine is, you probably know what your SDs will look like when ES is 50 or below.

I thought Ava sounded familiar, my uncle has a dairy farm and raises cattle not too far away from you in Oregon Co.;)

The Bushmaster
April 16, 2007, 01:13 AM
Yup...Have had the Pact 1 for around 16 years...:) Just needs a battery once in a while...:)

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