1. Are you sure the coefficients for x and x^2 are the same ?
2. If there is no intercept, it is unclear that R^2 is meaningful.
3. The RSSQ (and thus the MSE) must be smaller if the constant is
included.
At 12:44 +0000 05/12/2000, Homie wrote:
>I am currently running a simple quadratic model, with and without the
>constant (y=a+bx+bx^2) and (y=bx+bx^2).
>
>The model fit is much better when the constant is not included (see
>below). Although there is some minor theoretical reason for leaving
>out the constant, I am concerned that this would not be acceptable
>econometric practice.
>
>WITH CONSTANT
>Number of obs = 23
>F( 2, 20) = 13.77
>Prob > F = 0.0002
>R-squared = 0.5794
>Adj R-squared = 0.5373
>Root MSE = .08926
>
>WITHOUT CONSTANT
>Number of obs = 23
>F( 2, 21) = 44.90
>Prob > F = 0.0000
>R-squared = 0.8105
>Adj R-squared = 0.7924
>Root MSE = .08844
>
>Can Anyone explain:
>1. Why the model seems to fit better without the constant
>2. Is this an acceptable practice ? And how can you justify it.
>3. Are there any theoretical, mathematical drawbacks?
>
>Thanks
>
>
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