. Author manuscript; available in PMC: 2018 Dec 21.
Published in final edited form as: Biostatistics. 2019 Jan 1;20(1):1–16. doi: 10.1093/biostatistics/kxx058
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.