Household Income

Patients were grouped into four income quartiles from lowest to highest. Lower-income patients were generally older and had more comorbidity than higher-income patients. Income and education reflect different dimensions of socioeconomic position rather than being interchangeable measures. Income was averaged across the five years before the index date to reduce annual variation and short-term illness-related changes. Equivalent disposable household income was used as the study's marker of socioeconomic status.