  We analyze the dependence of the membership probabilities obtained from
kinematical variables on the radius of the field of view around open clusters
(the sampling radius, Rs). From simulated data, we show that the best
discrimination between cluster members and non-members is obtained when the
sampling radius is very close to the cluster radius. At higher Rs values more
field stars tend to be erroneously assigned as cluster members. From real data
of two open clusters (NGC 2323 and NGC 2311) we obtain that the number of
identified cluster members always increases with increasing Rs. However, there
is a threshold Rs value above which the identified cluster members are severely
contaminated by field stars and the effectiveness of membership determination
is relatively small. This optimal sampling radius is \sim 14 arcmin for NGC
2323 and \sim 13 arcmin for NGC 2311. We discuss the reasons for such behavior
and the relationship between cluster radius and optimal sampling radius. We
suggest that, independently of the method used to estimate membership
probabilities, several tests using different sampling radius should be
performed in order to evaluate the existence of possible biases.
