OnTarget TDS - SD Radius?!?!?!?!

[222Jim]

I'm looking for help understanding how OnTarget TDS calculates the "SD Radius" ("SD" standing for Standard Deviation"). Why? I've noticed that the SD Radius is always ridiculously large. Admittingly, I could simply be a horrible shooter shooting horrible ammunition and getting horrible groups. But that's not the situation.

To try to figure this out I drew up a dummy 8 shot target. On it I marked 4 "outer" bullet holes 1" from the center, and 4 "inner" bullet holes 0.5" from the center. Simple math says that's a 2.0" maximum spread, Mean Radius of 0.75", and Group Width and Height of 2.0". And, within the precision of my drawing skills, that's what OnTarget TDS reports.

BUT......the SD Mean Radius reported by OnTarget TDS is 0.792" vs. the 0.38" I calculate using my calculator and then my laptop (Excel).

So, anybody have any idea how OnTarget TDS caculates the SD Mean Radius.

 

[chromatica]

Try looking at the square root of the sum of the squares of each radius. This measurement is called RMS (Root Mean Square) and is often used in 2D error analysis. It has the effect of giving more weight to the larger errors.


So (1 + 1 + 1 + 1 + .25 + .25 + .25 + .25)/8 = .625. Square root of .625 = .79

 

[222Jim]

Try looking at the square root of the sum of the squares of each radius. This measurement is called RMS (Root Mean Square) and is often used in 2D error analysis. It has the effect of giving more weight to the larger errors.


So (1 + 1 + 1 + 1 + .25 + .25 + .25 + .25)/8 = .625. Square root of .625 = .79

Well that certainly explains it! I've used RMS in other engineering applications, but didn't think of trying it here.

Thanks