Questions about reading the numbers

[Spats McGee]

On the off chance that there was any confusion about the matter, let me be clear: I'm a Word Nerd. Mind you, I can do math, but if you ever catch me saying it's my strong suit, feel free to call me out, because I'd be lying. So, with my disclaimer out of the way, I have comments and a question about the following target. First, my comment. It says "55 gr on sale" at the center target, but I don't think that's right. I did use some 55 gr. on sale ammo, but I think the center target is 62 grain, something or other. I can't remember the brand right now, but that's neither here nor there. It also says 50 yards, but that's not right, either. This one was at 100. Clearly, I don't quite have my app figured out.

So here's my question: I see:
Group size: 1.22" (2.32 MOA); and
Mean radius: 0.48 (0.92 MOA)
Which one is "my MOA?" Did I shoot 2.32 or 0.92 MOA?

 

[trackskippy]

If the app is using the 50 yard distance, then it would be 2MOA. 1MOA is 1"at 100 yards +/-.

 

[jdsingleshot]

Both MOA numbers are correct for 50 yards). The overall group is 2.32 MOA. The mean radius is the length of the radius in MOA. (The mean radius means half the shots were of greater radius from the center of the group and half were of smaller radius from the center of the group.)

 

[Varminterror]

If the distance used in the app were correct, then the second number in “Group size,” the number in parentheses, is your “MOA” - meaning the angular dispersion of your group. Since you said the target was actually fired at 100 yards, and we can see the app is calculating based on 50yrds, then your group “MOA” is actually 1.16 MOA, half of the dispersion it calculated. 1.22” at 100yrds = 1.22/1.047 = 1.165 MOA.

The “mean radius” is the average distance of each shot from the calculated centroid of the group. So the relationship between the group size and the mean radius give you an idea about how good your group represents potential groups. In principle, statistically valid data sets should have mean radius be half of the group size, (half distance across the circle versus entire distance), but if we have a bunch of shots clustered together and a couple of flyers, like this group, we’ll have a group size which is larger than double the mean radius. This usually tells me if I shoot more shots, the group size will keep growing.

 

[Walkalong]

Heh, I still do it the old fashioned way. You’ll get it figured out. Then you can teach me.

 

[Dr T]

FWIW, I do not find the mean radius a particularly useful number other than as a qualitative measure of dispersion. It sort of feels like it's a "This is easy to calculate so let's just throw this in" sort of application/measurement feature. A true second order statistic (standard deviation) would be more useful. For me.

 

[trackskippy]

I always looked at MOA as being a "radius", especially as its related to POA.

And to me, an MOA group is basically useless/meaningless, if its not centered on POA.

 

[Spats McGee]

FWIW, I do not find the mean radius a particularly useful number other than as a qualitative measure of dispersion. It sort of feels like it's a "This is easy to calculate so let's just throw this in" sort of application/measurement feature. A true second order statistic (standard deviation) would be more useful. For me.

I get what you said about the mean radius. As to the bolded part, . . . Individually, I understand each of the words. In that combination, not so much.

 

[Walkalong]

All I'm interested in is group size and shape.

 

[C-grunt]

Some of those Apps are not very user friendly.

 

[Dr T]

As to the bolded part, . . . Individually, I understand each of the words. In that combination, not so much.

It is a mathematical fine point based on how distance is measured in a multidimensional vector spaces. Second order statistics are based on a fancy generalization of the Pythagorean theorem. From there, we have to go into Bell Curves, the heat equation, the Second Law of Thermodynamics, and Hilbert Spaces, and we do not have a keg of beer handy for the discussion. However, the fact that Second Law of Thermodynamics is one of the most import observations in science is one of my favorite topics...

 

[doubleh]

All I'm interested in is group size and shape.

Same here. For many years all I used was a machinest's scale to measure group size but then I bought a digital caliper and found it quicker and easier.