# Talk:Circular Error Probable

I think the section on "Systematic Accuracy Bias" misses the point.

Consider two experiments.

(1) Sight weapon then Shoot 10 groups

(2) (a)Sight weapon then shoot group (b) do (a) 10 times

Method (1) has accuracy bias but method (2) does not.

Herb (talk) 18:53, 1 June 2015 (EDT)

Here's my take: "Systematic accuracy bias" on this page means that the true center of the shot distribution is different from the point of aim (POA). The CEP-radius can then be calculated around two possible centers - either a) around the known POA, or b) around the unknown true distribution mean.
a) takes into account systematic accuracy bias because the further away the true center of the shot distribution is from the POA, the larger the CEP around the POA becomes, holding distribution spread constant. b) does not take into account systematic accuracy bias because CEP does not vary with increasing distance from true group center to POA, holding distribution spread constant.
In a) only distribution spread (or the full covariance matrix) has to be estimated from observed data, in b) the true center also has to be estimated.
Any help in getting this issue better across is highly appreciated.
The wiki in general uses a third point, the $$\overline{POI_{sample}}$$, from which the measurements are calculated. It is the only point for which an actual measurement can be obtained from the sample of shots.