A statistical method for the evaluation of the probable amount of interlopers included in the most recently identified asteroid families is developed. The method is based on the analysis of the local properties of the asteroidal distribution in the zones of the proper element space in which families are located. An application to the most important families listed by Zappal\`a \etal\ {\bf [1995, Icarus in press]} allows to derive the probable amount of interlopers at different size ranges. In some cases, the results suggest that the criteria adopted to derive the nominal memberships have been quite conservative. The procedure developed in this paper allows to assess ``how many'' interlopers are probably included in the nominal member lists, but not ``which ones''. However, such a quantitative estimate of the amount of probable interlopers is very useful and should be taken into account in any analysis of the physical properties of the families, since the presence of interlopers can explain some apparent discrepancies in several cases.