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Stata Code Fragment: 
Choosing Between Nonnested Models

The example here is taken from Chapter 7 of Greene's Econometric Analysis, Fourth Edition.

clear
input year    y       c  
1950   791.8   733.2   
1951   819.0   748.7   
1952   844.3   771.4   
1953   880.0   802.5   
1954   894.0   822.7   
1955   944.5   873.8   
1956   989.4   899.8   
1957  1012.1   919.7   
1958  1028.8   932.9   
1959  1067.2   979.4   
1960  1091.1  1005.1   
1961  1123.2  1025.2   
1962  1170.2  1069.0   
1963  1207.3  1108.4   
1964  1291.0  1170.6   
1965  1365.7  1236.4   
1966  1431.3  1298.9   
1967  1493.2  1337.7   
1968  1551.3  1405.9   
1969  1599.8  1456.7   
1970  1688.1  1492.0   
1971  1728.4  1538.8   
1972  1797.4  1621.9   
1973  1916.3  1689.6   
1974  1896.6  1674.0
1975  1931.7  1711.9
1976  2001.0  1803.9
1977  2066.6  1883.8
1978  2167.4  1961.0
1979  2216.2  2004.4
1980  2214.3  2000.4
1981  2248.6  2024.2
1982  2261.5  2050.7
1983  2334.6  2145.9
1984  2468.4  2239.9
1985  2509.0  2312.6
 end
gen c1 = c[_n-1]
gen y1 = y[_n-1]
reg c y y1
      Source |       SS       df       MS              Number of obs =      35
-------------+------------------------------           F(  2,    32) =10598.34
       Model |  7887172.12     2  3943586.06           Prob > F      =  0.0000
    Residual |  11907.0301    32  372.094689           R-squared     =  0.9985
-------------+------------------------------           Adj R-squared =  0.9984
       Total |  7899079.15    34  232325.857           Root MSE      =   19.29
------------------------------------------------------------------------------
           c |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
           y |   .8769845   .1068808     8.21   0.000     .6592755    1.094693
          y1 |   .0226182   .1087415     0.21   0.837    -.1988809    .2441174
       _cons |   10.17258   10.49558     0.97   0.340    -11.20622    31.55139
------------------------------------------------------------------------------
reg c y c1
      Source |       SS       df       MS              Number of obs =      35
-------------+------------------------------           F(  2,    32) =12544.66
       Model |  7889017.15     2  3944508.58           Prob > F      =  0.0000
    Residual |  10061.9955    32   314.43736           R-squared     =  0.9987
-------------+------------------------------           Adj R-squared =  0.9986
       Total |  7899079.15    34  232325.857           Root MSE      =  17.732
------------------------------------------------------------------------------
           c |      Coef.   Std. Err.      t    P>|t|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
           y |    .646071   .1041907     6.20   0.000     .4338414    .8583006
          c1 |   .2884832   .1185766     2.43   0.021     .0469506    .5300158
       _cons |   10.01303   9.450803     1.06   0.297    -9.237625    29.26369
------------------------------------------------------------------------------
nnest c  y y1 (y c1)
M1 : Y = a + Xb with X = [y y1]
M2 : Y = a + Zg with Z = [y c1]
J test for non-nested models
H0 : M1  t(31)       4.30087
H1 : M2  p-val       0.00016
H0 : M2  t(31)      -3.29065
H1 : M1  p-val       0.00250
Cox-Pesaran test for non-nested models
H0 : M1  N(0,1)    -28.34249
H1 : M2  p-val       0.00000
H0 : M2  N(0,1)      1.59375
H1 : M1  p-val       0.05550

According to J test, either model M1 or model M2 should be rejected. But according to Cox-Pesaran test, we shouldn't reject model M2 with the variables y and c1 and the predictors. Greene gave reference for discussion of why J test would reject both.


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