  Probability of coincidental clustering among the orbits of comets, asteroids
and meteoroids depends on many factors like: the size of the orbital sample
searched for clusters or the size of the identified group, it is different for
groups of 2,3,4, ... members. Probability of coincidental clustering is
assessed by the numerical simulation, therefore, it depends also on the method
used for the synthetic orbits generation.
  We tested the impact of some of these factors. For a given size of the
orbital sample we have assessed probability of random pairing among several
orbital populations of different sizes. We have found how these probabilities
vary with the size of the orbital samples.
  Finally, keeping fixed size of the orbital sample we have shown that the
probability of random pairing can be significantly different for the orbital
samples obtained by different observation techniques. Also for the user
convenience we have obtained several formulae which, for given size of the
orbital sample, one can use to calculate the similarity threshold corresponding
to the small value of the probability of coincidental similarity among two
orbits.
