1753 ARTIGO ARTICLE Georeferenced data in epidemiologic research Dados georreferenciados em epidemiologia Guilherme Loureiro Werneck 1,2 Abstract This paper reviews some conceptual Resumo Neste artigo, faz-se uma revisão acerca and practical issues regarding the application of de aspectos conceituais e práticos relacionados à georeferenced data in epidemiologic research. aplicação de dados georreferenciados na pesquisa Starting with the disease mapping tradition of epidemiológica. Iniciando com a tradicional abor- geographical medicine, topics such as types of geo- dagem de mapeamento de doenças da geografia referenced data, implications for data analysis, médica, discute-se heuristicamente com base em spatial autocorrelation and main analytical ap- exemplos da literatura epidemiológica, tópicos proaches are heuristically discussed, relying on como tipos de dados georreferenciados, implica- examples from the epidemiologic literature, most ções para a análise de dados, autocorrelação es- of them concerning mapping disease distribution, pacial e as principais estratégias analíticas, desta- detection of disease spatial clustering, evaluation cando-se os estudos de mapeamento da distribui- of exposure in environmental health investiga- ção espacial de eventos de saúde, detecção de agre- tion and ecological correlation studies. As for con- gados espaciais de casos, avaliação de exposição cluding remarks, special topics that deserve fur- em estudos de saúde ambiental e estudos ecológi- ther development, including the misuses of the cos. Os comentários finais salientam tópicos espe- concept of space in epidemiologic research, issues ciais que merecem desenvolvimentos futuros, in- related to data quality and confidentiality, the cluindo os dilemas relacionados à incorporação role of epidemiologic designs for spatial research, do conceito de espaço na pesquisa epidemiológica, sensitivity analysis and spatiotemporal modeling, aspectos relacionados à qualidade dos dados e con- are presented. fidencialidade, o papel dos estudos epidemiológi- Key words Epidemiology, Medical geography, cos na pesquisa com dados espaciais, análise de Epidemiologic methods, Small-area analysis, Eco- sensibilidade e modelos espaço-temporais. logical studies, Clustering Palavras-chave Epidemiologia, Geografia médi- 1 Departamento de Endemias Samuel Pessoa, Escola ca, Métodos epidemiológicos, Análise de pequenas Nacional de Saúde Pública, áreas, Estudos ecológicos, Conglomerados Fundação Oswaldo Cruz. Rua Leopoldo Bulhões 1480, Manguinhos. 21041-210 Rio de Janeiro RJ. [email protected] 2 Instituto de Medicina Social, Universidade do Estado do Rio de Janeiro. 1754 Werneck, G. L. G. Werneck, Introduction For nearly every epidemiologist, however, John Snow´s investigation of the cholera epidemic in Georeferenced data, also known as spatial, geo- London is the most famous work demonstrating graphical or geospatial data, are the basic pieces the importance of studying the geography of dis- of information needed to identify the geographic ease11. Considered a classical example of epide- location of phenomena across the Earth’s surface. miologic reasoning, which led to development of In general, georeferenced data consist of measure- a water-borne theory of cholera transmission, his ments or observations taken at specific locations work also became famous in medical geography (points referenced by latitude and longitude) or because he used a dot-map to plot the location of within specific regions (areal data). In epidemio- cholera deaths around the Broad Street pump in logic research, this type of information is mainly London Soho´s district. Nevertheless, the belief used to investigate the relationship between geo- that Snow used the dot-map to determine the referenced health events data and aspects related source of the cholera outbreak and to make a causal both to individual characteristics (e.g. genetic, be- connection between the removal of the pump and havioral and demographic) and contextual fac- the end of the outbreak seems not to be support- tors (e.g. socioeconomic neighborhood conditions, ed by evidence12. As a matter of fact, Snow already physical environment). Mapping disease distri- had his theory on the transmission of cholera bution, detection of disease spatial clustering, eval- before collecting data to test it13. uation of exposure in environmental health in- Despite the longstanding tradition of disease vestigation and ecological correlation studies are mapping and geographically oriented research, some examples of possible applications of geo- during the first half of the 20th century epidemi- referenced data in epidemiologic studies. ologists were more inclined to focus their research The geographical distribution of disease has on the time dimensions of disease distribution, been considered a key element in epidemiologic and progressively more and more on individual research, as indicated by the importance given to characteristics, a result of the rising emphasis on the description of health events according to “per- the biological causes of disease8,14. son, place and time” in the classic epidemiology Advances in geographic information systems textbooks1,2. In fact, studies on the geographical (GIS) permitted a remarkable increase in the effi- distribution of diseases come back to the 18th and ciency of processing and analysis of complex geo- 19th centuries, when the term “medical geogra- referenced data involving different variables at a phy” was devised3. variety of geographical scales, providing new tools Among the precursors of geographic studies for epidemiologists to incorporate place and space of disease are the physicians James Lind4, mainly in their investigations15. Since the 1970s, GIS and recognized for his work on scurvy, and Leon- related technologies, such as remote sensing, have hard Ludwig Finke5, who published in 1792 what spread rapidly to many scientific and technical has been considered the most detailed conceptu- fields, including public health and epidemiolo- al contribution on medical geography written to gy14-17. Today GIS and remote sensing are con- that point and the first to systematize world-wide sidered important tools in environmental health data5,6. A long tradition on disease mapping also research and disease surveillance15,18, and have started at that time. Aparently the first dot map been used to investigate patterns of disease spread applied to public health problems is due to Val- and inform control strategies for infectious dis- entine Seaman, in 1798, describing the distribu- eases, in particular vector-borne and zoonotic tion of yellow fever cases in New York7. diseases19,20, to help define the boundaries of the Early roots of epidemiology as a discipline can communities or neighborhoods where study par- be found at this time, with research focusing main- ticipants reside in multilevel studies21,22, and to ly on the relationship between societal conditions estimate socio-demographic and environmental and health8. Various researchers applied what variables21,23. This process of increasing incor- modern epidemiology usually calls ecological study poration of GIS in epidemiologic research should designs to address the problem of disease varia- not be considered simply as a result of techno- tion across different places. For instance, André- logical forces, but must be put in the context of Michel Guerry, in 1833, explored the variation of the renewed efforts detected after the 1960s to suicide and homicide rates across regions of integrate social sciences and epidemiology8. France9 and Engels, in 1892, cited evidence of vari- In this review, I intend to introduce the basic ation in mortality rates across different cities and approaches used by epidemiologists to deal with streets surrounding Manchester10. georeferenced data. I will focus on the strategies 1755 Ciência & Saúde Coletiva, 13(6):1753-1766, 2008 for analysis of this type of data, building upon ature could have been measured at any place, examples from different areas of epidemiologic hypothetically there are an infinite number of research. I wish to keep statistics apart as much places that you can place monitoring stations, as possible, emphasizing the conceptual issues but unfortunately you have, in general, only a behind the techniques. sample of possible locations to get data on tem- For the sake of completeness, it is necessary perature. On the other hand, a discrete spatial to mention that many authors use the terms “spa- domain means that you can count the number tial epidemiology” or “geographical epidemiolo- of locations in which observations are taken. An gy” to define this area of investigation24-28. None- example is the number of dengue cases recorded theless, I prefer not to support an unnecessary in neighborhoods of Rio de Janeiro in January autonomy to what I see as a field of epidemio- 2008. Here the spatial domain is discrete: you can logic practice that just carry within its roots a count the number of neighborhoods that con- criticsm concernig the excessive focus of modern figure the region of analysis. epidemiology on the individual causes of disease. A fixed spatial domain is the one that does not change from one realization of the spatial process to the next30. Heuristically, a realization of a spa- Types of georeferenced data tial process is simply a set of georeferenced obser- vations. Imagine you asked your research assis- Broadly speaking, geospatial data can be point tants to do a fieldwork using some kind