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While it's easy to assign data as yearly or quarterly in a time series analysis, when I use the date range function, I don't see a bi-yearly option, such as labeling data as August and January. Ultimately I would like to use the Expert Modeler and include seasonal models, but have been limited to non-seasonal models given that I can't figure this out. Thanks in advance for any help you can provide!

I'm afraid I don't understand what does this mean. Instead of describing the error could you describe what variables you have now and what kind of results you'd like to get?

I'm predicting college enrollment based on prior enrollment using time series modeling. When beginning the analysis, SPSS asks how to label the data -- yearly, quarterly, etc. There is no periodicity of 6 wherein I could enter August and January enrollment data, so up until now I've acted as though the data were yearly. The ARIMA (2,1,0) did pick up the relationship of data two time periods/lags apart, meaning Fall enrollment was related to prior Fall enrollment, and Winter was related to the Winter prior, but ultimately it seems as though there would be a way to enter semi-annual data and use seasonal models.

Could you use monthly but let the autoregressive order be 6?