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Math Help - Multiipe regression equation query

  1. #1
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    Multiipe regression equation query

    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
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  2. #2
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    Re: Multiipe regression equation query

    If i remember correctly you can either fit 2 seperate lines or a combined model as follows:

    y = frequency
    X1 = temperature
    X2 = socio-economic class
    X3 = Season indicator (0=spring/summer, 1=autumn/winter)

    y_i = a + b_1X_1 + b_2X_3X_1 + b_3X_2 + b_4X_3X_2 + b_5X3 +e_i

    This will give different slopes and intercepts for each value of the indicator variable. I dont remember, but i suspect the two approaches give the same answer unless you can use some process to reduce the number of terms in the combined model.
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  3. #3
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    Re: Multiipe regression equation query

    Thanks (again) Spring. You're fast becoming my MHF Hero!
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