The raw clinical data can be combined in many ways to producedifferent sets of diagnostic categories and diagnostic categories canin turn be combined in different ways to produce definitions ofmultimorbidity. A recent review (Huntley et al., 2012) found 17different multimorbidity classification systems ranging from sim-ple counts of the number of diagnoses, as in the Charlson system, toelaborate classification schemes such as the John Hopkins AdjustedClinical Groups (ACG) Case-Mix system (Johns Hopkins BloombergSchool of Public Health, 2008) and the Hierarchical Conditions Clas-sification (Pope et al., 2004).