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From   Matthew J Baker <matthew.baker@hunter.cuny.edu>
To   statalist@hsphsun2.harvard.edu
Date   Fri, 4 Feb 2011 09:53:21 -0500 (EST)

Dear listers:

I have noticed that there can be a (small) discrepancy between 
the results produced by Stata's predict command after running 
a regression and what I think should generate identical 
results in Mata. As an illustration, when running the 
following code:

  sysuse auto, clear
  reg price weight length
  predict xb
  gen err=price-xb

mata
  st_view(X=.,.,"weight length")
  X=X,J(rows(X),1,1)
  st_view(Y=.,.,"price")
  beta=invsym(X'X)*X'Y
  err_mata=Y-X*beta
  end
  getmata err_mata
  sum err err_mata

I find that the results of the sum command are as follows 
(apologies for bad spacing)
spacing): 

    Variable |       Obs        Mean    Std. Dev.       Min        
Max
-------------+------------------------------------------------
--------
         err |        74   -.0000198    2382.413  -4432.274    
5771.01
    err_mata |        74    6.20e-06    2382.413  -4432.274    
5771.01

One can see that the numbers are all close but a little 
different - notably, the mean. What is the reason for this? 

Dr. Matthew J. Baker
Department of Economics
Hunter College and the Graduate Center, CUNY
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