As strontium values are bound to geological formations the results can be less distinctive as we would like because the underground formations of a region can be quite homogeneous. Multiple isotope analyses of additional elements like – especially – oxygen improves the possibility to discriminate between locals and non-locals. This is illustrated by one immigrant at Swifterbant, which shows deviant Sr but local O values and two individuals at Schipluiden with normal Sr but deviant O values. Lead isotope analyses are in accordance with these findings and therefore strengthen the interpretation.
The combination of the various isotope studies on provenance and diet offer the possibility to elucidate inter-individual variety in diet as well. This is shown by the fact that some of the immigrants displayed a diet lower in fish. When relating these observations to the burial ritual it appears that all the people in the graves at Schipluiden were of local origin, and that both immigrants are represented by isolated molars only, bearing witness to an above-ground treatment of the dead. Differences with respect to gender cannot be studied as females are underrepresented in Schipluiden. A future isotope study of the Ypenburg population would be desirable and a valuable addition to this topic.