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Unformatted text preview: ASYMPTOTIC AND FINITESAMPLE DISTRIBUTIONS OF THE IV ESTIMATOR 1 u X Y + + = 2 1 ( 29 ( 29 ( 29 ( 29  = X X Z Z Y Y Z Z b i i i i IV 2 ( 29 ( 29 ( 29 ( 29 2 , 2 2 , 2 2 2 , 2 2 2 IV 2 1 MSD 1 1 1 var IV 2 Z X u Z X i u Z X i u b r X n r X X n n r X X b =  =  = = The asymptotic variance of the IV estimator is given by the expression shown. It is the expression for the variance of the OLS estimator, multiplied by the square of the reciprocal of the correlation between X and Z . 2 IV 2 plim = b ASYMPTOTIC AND FINITESAMPLE DISTRIBUTIONS OF THE IV ESTIMATOR 2 u X Y + + = 2 1 ( 29 ( 29 ( 29 ( 29  = X X Z Z Y Y Z Z b i i i i IV 2 ( 29 ( 29 ( 29 ( 29 2 , 2 2 , 2 2 2 , 2 2 2 IV 2 1 MSD 1 1 1 var IV 2 Z X u Z X i u Z X i u b r X n r X X n n r X X b =  =  = = What does this mean? We have seen that the distribution of the IV estimator degenerates to a spike. So how can it have an asymptotic variance? 2 IV 2 plim = b ASYMPTOTIC AND FINITESAMPLE DISTRIBUTIONS OF THE IV ESTIMATOR 3 u X Y + + = 2 1 ( 29 ( 29 ( 29 ( 29  = X X Z Z Y Y Z Z b i i i i IV 2 ( 29 ( 29 ( 29 ( 29 2 , 2 2 , 2 2 2 , 2 2 2 IV 2 1 MSD 1 1 1 var IV 2 Z X u Z X i u Z X i u b r X n r X X n n r X X b =  =  = = The contradiction has been caused by compressing several ideas together. We will have to unpick them, taking several small steps. 2 IV 2 plim = b ASYMPTOTIC AND FINITESAMPLE DISTRIBUTIONS OF THE IV ESTIMATOR 4 u X Y + + = 2 1 ( 29 ( 29 ( 29 ( 29  = X X Z Z Y Y Z Z b i i i i IV 2 ( 29 ( 29 ( 29 ( 29 2 , 2 2 , 2 2 2 , 2 2 2 IV 2 1 MSD 1 1 1 var IV 2 Z X u Z X i u Z X i u b r X n r X X n n r X X b =  =  = = The application of a central limit theorem (CLT) underlies the assertion. To use a CLT, we must first show that a variable has a nondegenerate limiting distribution. The CLT will then show that, under appropriate conditions, this limiting distribution is normal. 2 IV 2 plim = b ASYMPTOTIC AND FINITESAMPLE DISTRIBUTIONS OF THE IV ESTIMATOR 5 u X Y + + = 2 1 ( 29 ( 29 ( 29 ( 29  = X X Z Z Y Y Z Z b i i i i IV 2 ( 29 ( 29 ( 29 ( 29 2 , 2 2 , 2 2 2 , 2 2 2 IV 2 1 MSD 1 1 1 var IV 2 Z X u Z X i u Z X i u b r X n r X X n n r X X b =  =  = = We cannot apply a CLT to b 2 IV directly, because it does not have a nondegenerate limiting distribution. The expression for the variance may be rewritten as shown. MSD( X ) is the mean square deviation of X ....
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This note was uploaded on 05/26/2010 for the course ECON 301 taught by Professor Öcal during the Spring '10 term at Middle East Technical University.
 Spring '10
 öcal
 Econometrics

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