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Participants in this session will understand how Ipsos used GIS tools to confirm which environmental and demographic variables are directly related to the prevalence of Seasonal Affective Disorder (SAD). 13,359 people were surveyed. A strong linear relationship was found between SAD prevalence and latitude (seasonal variance in the number of hours of sunlight). A relationship was also found between SAD prevalence and the number of cloudy days, proximity to coastlines, and the respondent's city size. Demographic variables, age, gender, marital status, and employment were also found to have a direct relationship to SAD prevalence. Combining these variables into a model resulted in the predictors of prevalence. The most predictive environmental factor is the change in the number of hours of sunlight followed by experience of cloudy days. Proximity to a coast is more predictive of SAD prevalence than is the size of the city where the respondent lives.