Primary non-variceal upper gastrointestinal bleeding in NSAID and low-dose aspirin users: development and validation of risk scores for either medication in two large Dutch cohorts - de Groot, N L; Hagenaars, M P; Smeets, H M; Steyerberg, E W; Siersema, P D; van Oijen, M G (J Gastroenterol, 2014)