 This study found that the prevalence of multimorbidity varies greatly depending on the number and selection of conditions considered. When considering the two most common conditions, the prevalence rate was 4.6%, while when considering all 80 conditions, it was 40.5%. Additionally, the prevalence rate was much higher in older individuals than in younger ones, ranging from 29% in those over 80 years old to 71% in those under 9 years old. Furthermore, the prevalence rate also differed between condition lists, ranging from 11.1% to 36.4%. This suggests that a standardized approach to defining multimorbidity is necessary, and that researchers should use existing condition lists associated with the highest multimorbidity prevalence. This article was authored by Claire McRae, Megan McMen, Stuart W. Mercer, and others.