Examining variation in the measurement of multimorbidity in research: a systematic review of 566 studies.

Funding: This work was supported by supported by Health Data Research UK (HDR-UK; grant number CFC0110) which receives its funding from the UK Medical Research Council, Engineering and Physical Sciences Research Council, Economic and Social Research Council, Department of Health and Social Care (England), Chief Scientist Office of the Scottish Government Health and Social Care Directorates, Health and Social Care Research and Development Division (Welsh Government), Public Health Agency (Northern Ireland), British Heart Foundation, and the Wellcome Trust. .