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经济代写|宏观经济学作业代写Macroeconomics代考|The effects of mis-specification

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经济代写|宏观经济学作业代写Macroeconomics代考|The effects of mis-specification

经济代写|宏观经济学作业代写Macroeconomics代考|Under-parameterisation

Given the following DGP:
$$
\mathbf{y}=\mathbf{X}{1} \beta{1}+\mathbf{X}{2} \beta{2}+\mathbf{u}
$$
for which hypotheses $(1.12)-(1.20)$ hold, the following model is estimated :
$$
\mathbf{y}=\mathbf{X}{1} \beta{1}+\nu
$$
Therefore, the OLS estimates are given by the following expression

$$
\hat{\boldsymbol{\beta}}{1}^{u p}=\left(\mathbf{X}{1}^{\prime} \mathbf{X}{1}\right)^{-1} \mathbf{X}{1}^{\prime} \mathbf{y}
$$
while the OLS estimates which would have been obtained by estimation of the DGP would have been:
$$
\widehat{\boldsymbol{\beta}}{1}=\left(\mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{X}{1}\right)^{-1} \mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{y}
$$
The estimates in (1.39) are Best Linear Unbiased Estimators by construction, while the estimates in (1.38) are biased unless the correlation between $\mathbf{X}{1}$ and $\mathbf{X}{2}$ is zero. To show this point consider that:
$$
\begin{aligned}
\widehat{\boldsymbol{\beta}}{1} &=\left(\mathbf{X}{1}^{\prime} \mathbf{X}{1}\right)^{-1}\left(\mathbf{X}{1}^{\prime} \mathbf{y}-\mathbf{X}{1}^{\prime} \mathbf{X}{2} \widehat{\boldsymbol{\beta}}{2}\right) \ &=\widehat{\boldsymbol{\beta}}{1}^{u p}+\widehat{\mathbf{D}} \widehat{\boldsymbol{\beta}}{2} \end{aligned} $$ where $\widehat{\mathbf{D}}$ is the vector of coefficients in the regression of $\mathbf{X}{2}$ on $\mathbf{X}{1}$, and $\widehat{\boldsymbol{\beta}}{2}$ is the OLS estimator obtained by fitting the DGP.
To provide further interpretation of these results note that if we have:
$$
\begin{aligned}
E\left(\mathbf{y} \mid \mathbf{X}{1}, \mathbf{X}{2}\right) &=\mathbf{X}{1} \boldsymbol{\beta}{1}+\mathbf{X}{2} \boldsymbol{\beta}{2} \
E\left(\mathbf{X}{1} \mid \mathbf{X}{2}\right) &=\mathbf{X}{1} \mathbf{D} \end{aligned} $$ then $$ E\left(\mathbf{y} \mid \mathbf{X}{1}\right)=\mathbf{X}{1} \boldsymbol{\beta}{1}+\mathbf{X}{1} \mathbf{D} \boldsymbol{\beta}{2}=\mathbf{X}{1} \boldsymbol{\alpha} . $$ Therefore the OLS estimator in the underparameterised model is a biased estimator of $\beta{1}$, but it is an unbiased estimator of $\alpha$. Then, if the objective of the model is forecasting and $\mathbf{X}{1}$ is more easily observed than $\mathbf{X}{2}$, than the undeparameterised model can be safely used. On the other hand, if the objective of the model is to test specific predictions on parameters (as it is the case with the Solow’s growth model), than the use of the under-parameterised model will deliver biased results. When we are interested in the effect of $\mathbf{X}_{1}$ on $\mathbf{y}$, independently from other factors, it is crucial to control for the effects of omitted variables.

经济代写|宏观经济学作业代写Macroeconomics代考|Over-parameterization

Given the following DGP:
$$
\mathbf{y}=\mathbf{X}{1} \beta{1}+\mathbf{u}
$$
for which hypotheses $(1.12)-(1.20)$ hold, the following model is estimated :

$$
\mathbf{y}=\mathbf{X}{1} \boldsymbol{\beta}{1}+\mathbf{X}{2} \boldsymbol{\beta}{2}+\mathbf{v}
$$
The OLS estimator of the over-parameterized model is
$$
\widehat{\boldsymbol{\beta}}{1}^{o p}=\left(\mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{X}{1}\right)^{-1} \mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{y}
$$
while, by estimating the DGP, we obtain:
$$
\widehat{\boldsymbol{\beta}}{1}=\left(\mathbf{X}{1}^{\prime} \mathbf{X}{1}\right)^{-1} \mathbf{X}{1}^{\prime} \mathbf{y} .
$$
By substituting $y$ from the DGP it is immediately shown that both estimators are unbiased. The difference is now made by the variance. In fact we have:
$$
\begin{gathered}
\operatorname{var}\left(\widehat{\boldsymbol{\beta}}{1}^{o p} \mid \mathbf{X}{1}, \mathbf{X}{2}\right)=\sigma^{2}\left(\mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{X}{1}\right)^{-1} \
\operatorname{var}\left(\widehat{\boldsymbol{\beta}}{1} \mid \mathbf{X}{1}, \mathbf{X}{2}\right)=\sigma^{2}\left(\mathbf{X}{1}^{\prime} \mathbf{X}{1}\right)^{-1} \end{gathered} $$ It can be shown that the estimator derived from the correct model is more efficient. In fact, the difference between the two variance-covariance matrices is a positive semi-definite matrix. To show this result remember that if two matrices $\mathbf{A}$ and $\mathbf{B}$ are positive definite and $\mathbf{A}-\mathbf{B}$ is positive semi-definite, then also the matrix $\mathbf{B}^{-1}-\mathbf{A}^{-1}$ is positive semi-definite. Then we have to show that $\mathbf{X}{1}^{\prime} \mathbf{X}{1}-$ $\mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{X}{1}$ is a positive semi-definite matrix. Such result is almost immediately shown:
$$
\begin{aligned}
\mathbf{X}{1}^{\prime} \mathbf{X}{1}-\mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{X}{1} &=\mathbf{X}{1}^{\prime}\left(\mathbf{I}-\mathbf{M}{2}\right) \mathbf{X}{1} \
&=\mathbf{X}{1}^{\prime} \mathbf{Q}{2} \mathbf{X}{1}=\mathbf{X}{1}^{\prime} \mathbf{Q}{2} \mathbf{Q}{2} \mathbf{X}_{1}
\end{aligned}
$$
We can then conclude that overparameterization impact on the efficiency of estimators and on the power of tests of hypotheses.

经济代写|宏观经济学作业代写Macroeconomics代考|The effects of mis-specification

宏观经济学代写

经济代写|宏观经济学作业代写MACROECONOMICS代考|UNDER-PARAMETERISATION

给定以下 DGP:
$$
\mathbf{y}=\mathbf{X} {1} \beta {1}+\mathbf{X} {2} \beta {2}+\mathbf{u}
F这r在H一世CHH是p这吨H和s和s$(1.12)−(1.20)$H这ld,吨H和F这ll这在一世nG米这d和l一世s和s吨一世米一种吨和d:
\mathbf{y}=\mathbf{X} {1} \beta {1}+\nu
$$
因此,OLS 估计由以下表达式给出

$$
\hat{\boldsymbol{\beta}}{1}^{u p}=\left(\mathbf{X}{1}^{\prime} \mathbf{X}{1}\right)^{-1} \mathbf{X}{1}^{\prime} \mathbf{y}
$$
while the OLS estimates which would have been obtained by estimation of the DGP would have been:
$$
\widehat{\boldsymbol{\beta}}{1}=\left(\mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{X}{1}\right)^{-1} \mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{y}
$$
The estimates in (1.39) are Best Linear Unbiased Estimators by construction, while the estimates in (1.38) are biased unless the correlation between $\mathbf{X}{1}$ and $\mathbf{X}{2}$ is zero. To show this point consider that:
$$
\begin{aligned}
\widehat{\boldsymbol{\beta}}{1} &=\left(\mathbf{X}{1}^{\prime} \mathbf{X}{1}\right)^{-1}\left(\mathbf{X}{1}^{\prime} \mathbf{y}-\mathbf{X}{1}^{\prime} \mathbf{X}{2} \widehat{\boldsymbol{\beta}}{2}\right) \ &=\widehat{\boldsymbol{\beta}}{1}^{u p}+\widehat{\mathbf{D}} \widehat{\boldsymbol{\beta}}{2} \end{aligned} $$ where $\widehat{\mathbf{D}}$ is the vector of coefficients in the regression of $\mathbf{X}{2}$ on $\mathbf{X}{1}$, and $\widehat{\boldsymbol{\beta}}{2}$ is the OLS estimator obtained by fitting the DGP.
To provide further interpretation of these results note that if we have:
$$
\begin{aligned}
E\left(\mathbf{y} \mid \mathbf{X}{1}, \mathbf{X}{2}\right) &=\mathbf{X}{1} \boldsymbol{\beta}{1}+\mathbf{X}{2} \boldsymbol{\beta}{2} \
E\left(\mathbf{X}{1} \mid \mathbf{X}{2}\right) &=\mathbf{X}{1} \mathbf{D} \end{aligned} $$ then $$ E\left(\mathbf{y} \mid \mathbf{X}{1}\right)=\mathbf{X}{1} \boldsymbol{\beta}{1}+\mathbf{X}{1} \mathbf{D} \boldsymbol{\beta}{2}=\mathbf{X}{1} \boldsymbol{\alpha} . $$ Therefore the OLS estimator in the underparameterised model is a biased estimator of $\beta{1}$, but it is an unbiased estimator of $\alpha$. Then, if the objective of the model is forecasting and $\mathbf{X}{1}$ is more easily observed than $\mathbf{X}{2}$, than the undeparameterised model can be safely used. On the other hand, if the objective of the model is to test specific predictions on parameters (as it is the case with the Solow’s growth model), than the use of the under-parameterised model will deliver biased results. When we are interested in the effect of $\mathbf{X}_{1}$ on $\mathbf{y}$, 独立于其他因素,控制遗漏变量的影响至关重要。

经济代写|宏观经济学作业代写MACROECONOMICS代考|OVER-PARAMETERIZATION

给定以下 DGP:
$$
\mathbf{y}=\mathbf{X} {1} \beta {1}+\ mathbf {u}
$$
(1.12)−(1.20)持有,估计以下模型:

$$
\mathbf{y}=\mathbf{X}{1} \boldsymbol{\beta}{1}+\mathbf{X}{2} \boldsymbol{\beta}{2}+\mathbf{v}
$$
The OLS estimator of the over-parameterized model is
$$
\widehat{\boldsymbol{\beta}}{1}^{o p}=\left(\mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{X}{1}\right)^{-1} \mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{y}
$$
while, by estimating the DGP, we obtain:
$$
\widehat{\boldsymbol{\beta}}{1}=\left(\mathbf{X}{1}^{\prime} \mathbf{X}{1}\right)^{-1} \mathbf{X}{1}^{\prime} \mathbf{y} .
$$
By substituting $y$ from the DGP it is immediately shown that both estimators are unbiased. The difference is now made by the variance. In fact we have:
$$
\begin{gathered}
\operatorname{var}\left(\widehat{\boldsymbol{\beta}}{1}^{o p} \mid \mathbf{X}{1}, \mathbf{X}{2}\right)=\sigma^{2}\left(\mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{X}{1}\right)^{-1} \
\operatorname{var}\left(\widehat{\boldsymbol{\beta}}{1} \mid \mathbf{X}{1}, \mathbf{X}{2}\right)=\sigma^{2}\left(\mathbf{X}{1}^{\prime} \mathbf{X}{1}\right)^{-1} \end{gathered} $$ It can be shown that the estimator derived from the correct model is more efficient. In fact, the difference between the two variance-covariance matrices is a positive semi-definite matrix. To show this result remember that if two matrices $\mathbf{A}$ and $\mathbf{B}$ are positive definite and $\mathbf{A}-\mathbf{B}$ is positive semi-definite, then also the matrix $\mathbf{B}^{-1}-\mathbf{A}^{-1}$ is positive semi-definite. Then we have to show that $\mathbf{X}{1}^{\prime} \mathbf{X}{1}-$ $\mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{X}{1}$ is a positive semi-definite matrix. Such result is almost immediately shown:
$$
\begin{aligned}
\mathbf{X}{1}^{\prime} \mathbf{X}{1}-\mathbf{X}{1}^{\prime} \mathbf{M}{2} \mathbf{X}{1} &=\mathbf{X}{1}^{\prime}\left(\mathbf{I}-\mathbf{M}{2}\right) \mathbf{X}{1} \
&=\mathbf{X}{1}^{\prime} \mathbf{Q}{2} \mathbf{X}{1}=\mathbf{X}{1}^{\prime} \mathbf{Q}{2} \mathbf{Q}{2} \mathbf{X}_{1}
\end{aligned}
$$
然后我们可以得出结论,过度参数化会影响估计器的效率和假设检验的能力。

经济代写|宏观经济学作业代写Macroeconomics代考

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