Figure 1. Causal directed acyclic graph.

The direct and indirect pathways between ethnicity and chlamydia diagnosis are shown. Three main routes were identified through which ethnicity could be associated with chlamydia: (1) a direct biological route, (2) a direct route through sexual risk behaviour, and (3) an indirect route through socio-economic status. Age and gender were identified as possible confounders of the association between ethnicity and chlamydia, sexual risk behaviour and chlamydia, and socio-economic status and chlamydia. Ethnic mixing was assumed as a possible confounder of the association between sexual risk behaviour and chlamydia.