Spatial and space-time scan statistics to detect low rate clusters of sex ratio JEAN-FRANCOIS VIEL*, NATHALIE FLORET and FREDERIC MAUNY Biostatistics and Epidemiology Unit, Department of Public Health, Faculty of Medicine, place Saint Jacques, 25030 Besanc ¸ on, France E-mail: jean-francois.viel@ufc-chu.univ-fcomte.fr Whether general environmental exposures to endocrine disrupting chemicals (including pes- ticides and dioxin) might induce decreased sex ratios (male/female ratio at birth) is discussed. To address this issue, the authors looked for a space-time clustering test which could detect local areas of significantly low risk, assuming a Bernoulli distribution. As a matter of fact, if the endocrine disruptor hypothesis holds true, and if the sex ratio is a sentinel health event indicative of new reproductive hazards ascribed to environmental factors, then in a given region, either a cluster of low male/female ratio among newborn babies would be expected in the vicinity of polluting municipal solid waste incinerators (MSWIs) (supporting the dioxin hypothesis), or local clusters would be expected in some rural areas where large amounts of pesticides are sprayed. Among cluster detection tests, the spatial scan statistic has been widely used in various applications to scan for areas with high rates, and rarely (if ever) with low rates. Therefore, the goal of this paper was to check the properties of the scan statistics under a given scenario (Bernoulli distribution, search for clusters with low rates) and to assess its added value in addressing the sex ratio issue. This study took place in the Franche-Comte´ region (France), mainly rural, comprising three main MSWIs, among which only one had high dioxin emissions level in the past. The study population consisted of 192,490 boys and 182,588 girls born during the 1975–1999 period. On the whole, the authors conclude that: (i) spatial and space-time scan statistics provide attractive features to address the sex ratio issue; (ii) sex ratio is not markedly affected across space and does not provide a reliable screening measure for detecting reproductive hazards ascribed to environmental factors. Keywords: Bernoulli distribution, clusters, endocrine disruptors, scan statistics, sex ratio, spatial epidemiology 1352-8505 Ó 2005 Springer Science+Business Media, Inc. 1. Introduction The analysis of disease clustering has generated considerable interest in the area of public health surveillance, and subsequently a large number of different tests has been proposed. According to Besag and Newell’s classification, one can distinguish *Corresponding author Environmental and Ecological Statistics 12, 289–299, 2005 1352-8505 Ó 2005 Springer Science+Business Media, Inc.