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st: problem with squared term


From   Prabhat <prabhat.barnwal@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: problem with squared term
Date   Sun, 21 Mar 2010 02:42:19 +0900

Dear all,

I have come across a strange problem. I am trying to estimate the
coefficients for temperature and rainfall using WLS, where my
dependent variable is rice yield.
Now, when I am including the square term for the average temperature,
I am getting a very high and impossible estimate for the temperature
variable. It should be somewhere between 100-300, but after including
square term I am getting -3500.
I have just included OLS results here.

Any comment will be appreciated.


Regards,
Prabhat Barnwal
International University of Japan


> . regress kyrice kavgtemp kavgtempsq ktotppt ktotpptsq ksdtemp

      Source |       SS       df       MS              Number of obs =     735
-------------+------------------------------           F(  5,   729) =   13.16
       Model |  21551254.8     5  4310250.95           Prob > F      =  0.0000
    Residual |   238840813   729  327628.001           R-squared     =  0.0828
-------------+------------------------------           Adj R-squared =  0.0765
       Total |   260392067   734  354757.585           Root MSE      =  572.39

------------------------------------------------------------------------------
      kyrice |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
    kavgtemp |  -3591.648   855.2294    -4.20   0.000    -5270.655   -1912.642
  kavgtempsq |   67.01803   15.34115     4.37   0.000     36.89993    97.13613
     ktotppt |   1.053627   .5836936     1.81   0.071    -.0922936    2.199548
   ktotpptsq |  -.0005872   .0003753    -1.56   0.118     -.001324    .0001496
     ksdtemp |  -196.4774   56.57082    -3.47   0.001    -307.5386   -85.41624
       _cons |    49759.5   11888.92     4.19   0.000      26418.9     73100.1
------------------------------------------------------------------------------
-> . regress kyrice kavgtemp  ktotppt ktotpptsq ksdtemp

      Source |       SS       df       MS              Number of obs =     735
-------------+------------------------------           F(  4,   730) =   11.39
       Model |  15298828.3     4  3824707.07           Prob > F      =  0.0000
    Residual |   245093239   730  335744.163           R-squared     =  0.0588
-------------+------------------------------           Adj R-squared =  0.0536
       Total |   260392067   734  354757.585           Root MSE      =  579.43

------------------------------------------------------------------------------
      kyrice |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
    kavgtemp |   143.2145   22.12018     6.47   0.000     99.78776    186.6413
     ktotppt |   .9560253   .5904461     1.62   0.106    -.2031498      2.1152
   ktotpptsq |    -.00057   .0003799    -1.50   0.134    -.0013158    .0001758
     ksdtemp |  -213.2879   57.13459    -3.73   0.000    -325.4556   -101.1202
       _cons |  -2107.959   622.1736    -3.39   0.001    -3329.422   -886.4957
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