@article{Heenan1965,
author = {Heenan, L.D.B.},
title= {The population pyramid: A versatile research technique},
year = {1965},
doi = {10.1111/j.0033-0124.1965.00018.x},
journal = {The Professional Geographer},
volume = {17},
number = {2},
pages = {18‒21},
}