Suppose I'm looking at frequency of domestic garden watering and through running a t-test have determined that there are two periods in time which differ significantly in garden watering frequency - spring/summer and autumn/winter. I've also run correlations and determined that the key predictor variables in both cases are temperature and socio-economic status of the household.

Now if I want to perform a multivariate regression to predict frequency of garden watering, do I need to create two regression equations, one for spring/summer and one for autumn/winter given that the t-test showed the distributions are different, or do I just create one regression equation for the whole year?

Thanks

-Rob