used to test whether there is horizontal equity in the allocationof primary care resources: after allowing for need, cost should notvary with a patient’s socio-economic status (Gravelle et al., 2006;Bago d’Uva, 2005). But if the data on patient morbidity are poor, anyassociation between socio-economic status and healthcare costsmay not be due to horizontal inequity but to the correlation ofsocio-economic status and unobserved aspects of morbidity.