Authors ignored data in their study

EDITOR—I take issue with Kennedy et al's conclusion that the frequently observed association between income inequality and health is not an artefact of an omitted relation between individual income and individual health.1 They had only limited information on individual incomes in their study. Furthermore, they ignored some of the income and inequality data that they did have available. Consequently, their conclusion that income inequality exerts an independent effect on individual health is inappropriate.

The authors controlled for individual income by using survey data on household income and controlling for household size. Their income data were categorical and so were necessarily measured with error. The top category was for households with incomes of ≥$50 000 (17.3% of the sample). Without explanation the authors combined this category with the category of households with an income of $35 000 to <$50 000 (another 15% of the sample). The authors also retained observations on people for whom …