We show that a 2D projection is representative of its corresponding 3D distribution at a confidence level of 90 \% if it follows a King profile and if we consider the whole spatial distribution .
The level is significantly lower and not decisive in the vicinity of the 2D cluster center .
On another hand , if we verify the reciprocal statement of the Mattig ’ s distribution ( 1958 ) -i.e .
a flux limited sample is represented by a 0.6 slope of its count law- , we point out that , due to the usual unaccuracy of the slope determination , a slope of 0.6 is not a sufficiently strict criterion for completeness and uniformity of a sample as often used in the literature .