K Abuabara1, E J van Zuuren2, C Flohr3. 1. Program for Clinical Research, Department of Dermatology, University of California San Francisco (UCSF), San Francisco, CA, U.S.A. 2. Leiden University Medical Centre, Department of Dermatology, Leiden, the Netherlands. 3. Unit for Population-Based Dermatology Research, St John's Institute of Dermatology, King's College London and Guy's and St Thomas' NHS Foundation Trust, London, U.K.