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Dear allstatters I would appreciate your advice, as I have been unable to find a specific answer to the following: How does SPSS 'know' which dummy variables belong together as a group coding for one nominal variable, and which belong to another? I am aware there may be an obvious answer to this question, please let me know if so. I am in the process of performing multiple regression analysis looking at levels of a particular cell type in four different groups of patients (early arthritis/reactive arthritis/remission/controls). Other IVs include age, CRP, disease duration and gender. I have created 3 dummy variables, coding 1/0 (yes/no) for the three types of arthritis patient, and it's my understanding that the controls would then act as the reference category. However, since gender is also coded 1/0 (male/female), how does this affect the patient group dummy variables? How does SPSS 'know' which patients belong to the control group (which contains those patients not coded 'yes' by any of the three patient group dummies) when there is another binary variable in the same data set? Thanks for your time Liz Hensor Dr Elizabeth M A Hensor PhD BSc (Hons) Data Analyst Academic Unit of Musculoskeletal and Rehabilitation Medicine 36 Clarendon Road Leeds West Yorkshire LS2 9NZ Tel: +44 (0) 113 3434933 Fax: +44 (0) 113 2430366 [log in to unmask]