  One of the main problems in the studies of large scale galaxy structures
concerns the relation of the correlation properties of a certain population of
objects with those of a selected subsample of it, when the selection is
performed by considering physical quantities like luminosity or mass. I
consider the case where the sampling is defined as in the simplest thresholding
selection scheme of the peaks of a Gaussian random field as well as the case of
the extraction of point distributions in high density regions from
gravitational N-body simulations. I show that an invariant scale under sampling
is represented by the zero-crossing scale of \xi(r). By considering recent
measurements in the 2dF and SDSS galaxy surveys I note that the zero-point
crossing length has not yet been clearly identified, while a dependence on the
finite sample size related to the integral constraint is manifest. I show that
this implies that other length scales derived from \xi(r) are also affected by
finite size effects. I discuss the theoretical implications of these results,
when considering the comparison of structures formed in N-body simulations and
observed in galaxy samples, and different tests to study this problem.
