title: [Internal validity in cross-sectional studies: comments based on an investigation on the association between socioeconomic factors and schistosomiasis].
abstract: The purpose of this paper was to make some comments on validity based on an investigation developed in an urban area in the Greater Metropolitan Area of Recife, Brazil, the objective of which was to study the association between socioeconomic factors and schistosomiasis. It discusses the distortion in the odds ratio that may be introduced either by the way individuals are selected for the study or by the way the information is collected. The article describes the mechanisms used by the authors to avoid bias when planning or conducting the investigation and considerations were made on how these types of bias could have influenced the results.
authors.affiliation: Departamento de Medicina Tropical, Hospital das Clínicas, Universidade Federal de Pernambuco, Recife, PE, 50670-420, Brasil.
authors.forename: R A
authors.initials: RA
authors.lastname: Ximenes
authors.forename: T V
authors.initials: TV
authors.lastname: Araújo