Keyfitz, N. <https://pure.iiasa.ac.at/view/iiasa/2026.html>  (1985).  Heterogeneity and Selection in Population Analysis.    In:  Applied Mathematical Demography.  pp. 385-399 New Yok, USA: Springer.  ISBN 978-1-4757-1879-9 10.1007/978-1-4757-1879-9_14 <https://doi.org/10.1007/978-1-4757-1879-9_14>.