Variation in the prevalence of helminths. Variation in the prevalence of helminths (all species combined) (A), hookworms (B) and Giardia duodenalis (C) by region of origin of subjects and residency status. The minimum sufficient models for A, B and C were LR χ2158 = 133.3 (P = 0.9), LR χ2158 = 111.1 (P = 0.998), and LR χ2152 = 95.7 (P = 1.0), respectively. All models included additional terms which did not incorporate prevalence of parasites. For significance of the illustrated effects see text.