Circular error probable (CEP), also circular error probability or circle of equal probability, is a measure of a weapon system's precision in the military science of ballistics. It is defined as the radius of a circle, centered on the aimpoint, that is expected to enclose the landing points of 50% of the rounds; said otherwise, it is the median error radius. That is, if a given munitions design has a CEP of 100 m, when 100 munitions are targeted at the same point, an average of 50 will fall within a circle with a radius of 100 m about that point.
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CEP concept and hit probability. 0.2% outside the outmost circle.
I didn't know what this was, so:
https://en.wikipedia.org/wiki/Circular_error_probable
Please note I was pulling most of the numbers out of my ass to illustrate the concept.
Oh I didn't take those literally, I'd just never heard of CEP and I figured I wasn't alone!