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How to account for population size in a poisson regression on SPSS

  • 1.  How to account for population size in a poisson regression on SPSS

    Posted Mon January 16, 2023 12:39 PM

    Hello everyone. 

    I am struggling to conduct a poisson regression with the following data: 

    My outcome variable is the number of self-harm incidents per patient (n=15) on a hospital, further divided into these categories for each patient: ethnicity, level of care (medium, enhanced) and diagnosis. I want to find out if these categories influence the frequency of the self-harm incidents, and therefore thought of conducting a poisson regression with count of self-harm incidents as my outcome variable and ethnicity, diagnosis and level of care as predictors. My only issue is that there are on average more patients in the enhanced level of care, and more white patients. I want this population size difference to be reflected in my analysis to make sure the effect of the predictors are estimated properly. However, I'm not sure how to do this in SPSS. Should this be added as an offset variable? I've only seen it being done to take into account the time of exposure (observational data of varying amount of time per participant) and not population size. Would an offset variable taking into account population size be the log of the total amount of patients in each level of care? Additionally, because I have 2 varying populations sizes (ethnicity and level of care), how can I include two offset variables in my model? Is there any other way to conduct this analysis (using an other model)? Any help would be greatly appreciated! Please let me know if anything is unclear 



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    Nathan
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