@article{Maas2005,
author = {Maas, C.J.M. and Hox, J.J.},
title= {Sufficient sample sizes for multilevel modeling},
year = {2005},
doi = {10.1027/1614-2241.1.3.85},
journal = {Methodology},
volume = {1},
number = {3},
pages = {86--92},
}