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Re: st: Why do I get two different results from the same specification and the same dataset?


From   Yuval Arbel <yuval.arbel@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Why do I get two different results from the same specification and the same dataset?
Date   Sun, 6 Nov 2011 20:16:40 +0200

Thanks for your assistance. Indeed there is a mistake:

I mistakenly defined dev_cost_int as dev_cost*area instead of dev_cost*min

Yuval

On Sun, Nov 6, 2011 at 7:03 PM, Richard Williams
<richardwilliams.ndu@gmail.com> wrote:
> Like everyone else, I strongly suspect you computed the interactions wrong.
> Maybe you accidentally used a wrong variable in the calculation. I would
> especially double-check the calculation of the units interaction, as it is
> by far and away the most different coefficient in your two models: 128,569.4
> in the first model (and highly significant) and -.2338972 (and
> insignificant) in the 2nd. Also the main effect of units switches from large
> positive to large negative.
>
> If you are convinced the coding is right I might try rescaling bid_win, e.g.
> dividing by 1000. Their might be some sort of precision issue.
>
> At 07:24 AM 11/6/2011, Yuval Arbel wrote:
>>
>> Dear statalist participants,
>>
>> when I run the following regression
>>
>> reg bid_win dev_cost bid_num year area units min min_price
>> c.dev_cost#i.min c.bid_num#i.min c.year#i.min c.area#i.min
>> c.units#i.min
>>
>> I get the following output:
>>
>>
>>      Source |       SS       df       MS              Number of obs =
>>  6802
>> -------------+------------------------------           F( 12,  6789) =
>> 2891.19
>>       Model |  7.0107e+17    12  5.8423e+16           Prob > F      =
>>  0.0000
>>    Residual |  1.3719e+17  6789  2.0207e+13           R-squared     =
>>  0.8363
>> -------------+------------------------------           Adj R-squared =
>>  0.8361
>>       Total |  8.3826e+17  6801  1.2326e+14           Root MSE      =
>>  4.5e+06
>>
>>
>> ------------------------------------------------------------------------------
>>     bid_win |      Coef.   Std. Err.      t    P>|t|     [95% Conf.
>> Interval]
>>
>> -------------+----------------------------------------------------------------
>>    dev_cost |  -.0782451   .1319286    -0.59   0.553    -.3368666
>>  .1803764
>>     bid_num |   53637.98   18087.12     2.97   0.003     18181.54
>>  89094.41
>>        year |   61991.65   44544.85     1.39   0.164    -25330.21
>>  149313.5
>>        area |   204.3705   105.0742     1.95   0.052    -1.607834
>>  410.3488
>>       units |   52691.04   11756.39     4.48   0.000     29644.82
>>  75737.26
>>         min |   1.04e+08   9.74e+07     1.06   0.288    -8.73e+07
>>  2.95e+08
>>   min_price |   3.956053   .0241168   164.04   0.000     3.908777
>> 4.00333
>>             |
>>         min#|
>>  c.dev_cost |
>>          1  |   -.460682   .1391518    -3.31   0.001    -.7334631
>>  -.187901
>>             |
>>         min#|
>>   c.bid_num |
>>          1  |  -43194.68   19552.09    -2.21   0.027     -81522.9
>> -4866.457
>>             |
>>  min#c.year |
>>          1  |  -51639.35   48543.57    -1.06   0.287      -146800
>>  43521.27
>>             |
>>  min#c.area |
>>          1  |   186.6343   110.1857     1.69   0.090    -29.36416
>>  402.6327
>>             |
>>  min#c.units |
>>          1  |  -128569.4   12642.23   -10.17   0.000    -153352.2
>> -103786.7
>>             |
>>       _cons |  -1.25e+08   8.94e+07    -1.39   0.163    -3.00e+08
>>  5.05e+07
>>
>> ------------------------------------------------------------------------------
>>
>>
>> But when I define directly the interaction variables, and run the
>> regression, I get different outcomes:
>>
>> . reg bid_win dev_cost bid_num year area units min min_price
>> dev_cost_int bid_num_int year_int area_int units_int
>>
>>      Source |       SS       df       MS              Number of obs =
>>  6802
>> -------------+------------------------------           F( 12,  6789) =
>> 2840.90
>>       Model |  6.9905e+17    12  5.8254e+16           Prob > F      =
>>  0.0000
>>    Residual |  1.3921e+17  6789  2.0505e+13           R-squared     =
>>  0.8339
>> -------------+------------------------------           Adj R-squared =
>>  0.8336
>>       Total |  8.3826e+17  6801  1.2326e+14           Root MSE      =
>>  4.5e+06
>>
>>
>> ------------------------------------------------------------------------------
>>     bid_win |      Coef.   Std. Err.      t    P>|t|     [95% Conf.
>> Interval]
>>
>> -------------+----------------------------------------------------------------
>>    dev_cost |   .3458744   .1259233     2.75   0.006     .0990254
>>  .5927235
>>     bid_num |   50612.77   18218.04     2.78   0.005     14899.71
>>  86325.84
>>        year |    26138.3   44731.32     0.58   0.559     -61549.1
>>  113825.7
>>        area |    796.392   88.98841     8.95   0.000     621.9468
>>  970.8371
>>       units |  -56322.78   4522.886   -12.45   0.000    -65189.06
>> -47456.51
>>         min |   2.11e+07   9.78e+07     0.22   0.829    -1.71e+08
>>  2.13e+08
>>   min_price |   3.914549   .0241269   162.25   0.000     3.867252
>>  3.961845
>> dev_cost_int |  -.9575921   .1316807    -7.27   0.000    -1.215728
>> -.6994567
>>  bid_num_int |  -40191.13   19694.51    -2.04   0.041    -78798.54
>> -1583.728
>>    year_int |  -10450.87   48755.43    -0.21   0.830    -106026.8
>>  85125.05
>>    area_int |  -443.5883   91.15801    -4.87   0.000    -622.2866
>> -264.89
>>   units_int |  -.2338972    .131622    -1.78   0.076    -.4919176
>>  .0241233
>>       _cons |  -5.29e+07   8.98e+07    -0.59   0.556    -2.29e+08
>>  1.23e+08
>>
>> ------------------------------------------------------------------------------
>>
>> My question is why do I get two different results from the same
>> specification?
>> Just to exemplify: note that the coefficient of "dev_cost" has
>> modified signs and became significant
>>
>> --
>> Dr. Yuval Arbel
>> School of Business
>> Carmel Academic Center
>> 4 Shaar Palmer Street, Haifa, Israel
>> e-mail: yuval.arbel@gmail.com
>> *
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>> *   http://www.ats.ucla.edu/stat/stata/
>
> -------------------------------------------
> Richard Williams, Notre Dame Dept of Sociology
> OFFICE: (574)631-6668, (574)631-6463
> HOME:   (574)289-5227
> EMAIL:  Richard.A.Williams.5@ND.Edu
> WWW:    http://www.nd.edu/~rwilliam
>
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
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> *   http://www.ats.ucla.edu/stat/stata/
>



-- 
Dr. Yuval Arbel
School of Business
Carmel Academic Center
4 Shaar Palmer Street, Haifa, Israel
e-mail: yuval.arbel@gmail.com

*
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