1. Department of Biostatistics, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD 21205–3179, USA. E‐mail:.
2. Departments of Biostatistics and Environmental Health, School of Public Health and Community Medicine, University of Washington, Seattle, WA, USA. E‐mail:.
3. Merlise Clyde, Institute of Statistics and Decisions Sciences, Duke University, Durham, NC 27708–0251, USA. E‐mail: