- December 22, 2020
- Uncategorized

## what are robust standard errors

There is much to think about before using robust standard errors. I have read a lot about the pain of replicate the easy robust option from STATA to R to use robust standard errors. In our simple model above, \(k = 2\), since we have an intercept and a slope. Before we do that, let’s use this formula by hand to see how it works when we calculate the usual standard errors. The s2 object above is the estimated variance of that Normal distribution. Charles. Hence, obtaining the correct SE, is critical We see then that H3 is a ratio that will be larger for values with high residuals and relatively high hat values. Robust standard errors The regression line above was derived from the model savi = β0 + β1inci + ϵi, for which the following code produces the standard R output: # Estimate the model model <- lm (sav ~ inc, data = saving) # Print estimates and standard test statistics summary (model) We then check how often we correctly reject the null hypothesis of no interaction between x and g. This is an estimation of power for this particular hypothesis test. Each estimate is again the square root of the elements of the diagonal of the covariance matrix as described above, except that we use a different version of S. Here, the hi are the leverage values (i.e. Recall that: 1. When robust standard errors are employed, the numerical equivalence between the two breaks down, so EViews reports both the non-robust conventional residual and the robust Wald F-statistics. To make this easier to demonstrate, we’ll use a small toy data set. In the most general case where all errors are correlated with each other, Stata Statistical Software: Release 16. A Google search or any textbook on linear modeling can tell you more about hat values and how they’re calculated. “vce” is short for “variance-covariance matrix of the estimators”. ”Robust” standard errors is a technique to obtain unbiased standard errors of OLS coefficients under heteroscedasticity. where \(h_i\) are the hat values from the hat matrix. Stata 16 Base Reference Manual. Please let me know if I’m doing something wrong. Key Concept 15.2 HAC Standard errors Problem: Standard errors based on this procedure are called (heteroskedasticity) robust standard errors or White-Huber standard errors. The overall fit is the same as standard OLS and coefficients are the same but standard error is different? “robust” indicates which type of variance-covariance matrix to calculate. -xtreg- with fixed effects and the -vce(robust)- option will automatically give standard errors clustered at the id level, whereas -areg- with -vce(robust)- gives the non-clustered robust standard errors. As you can see from Figure 2, the only coefficient significantly different from zero is that for Infant Mortality. However, autocorrelated standard errors render the usual homoskedasticity-only and heteroskedasticity-robust standard errors invalid and may cause misleading inference. The usual method for estimating coefficient standard errors of a linear model can be expressed with this somewhat intimidating formula: \[\text{Var}(\hat{\beta}) = (X^TX)^{-1} X^T\Omega X (X^TX)^{-1}\] where \(X\) is the model matrix (ie, the matrix of the predictor values) and \(\Omega = \sigma^2 I_n\), which is shorthand for a matrix with nothing but \(\sigma^2\) on the diagonal and 0’s everywhere else. Heteroskedasticity just means non-constant variance. Below s2 is \(\sigma^2\), diag(5) is \(I_n\), and X is the model matrix. The resulting standard error for ̂ is often called a robust standard error, though a better, more precise term, is heteroskedastic-robust standard error. From testing my data was found to be heteroscedastic. HC4 is a more recent approach that can be superior to HC3. URL, R Core Team (2020). The estimated variance is instead the residual squared multiplied by (5/3). 1. Real Statistics Function: The following array function computes the coefficients and their standard errors for weighted linear regression. The proportion of times we reject the null of no interaction using robust standard errors is lower than simply using the usual standard errors, which means we have a loss of power. Clearly the 5th data point is highly influential and driving the “statistical significance”, which might lead us to think we have specified a “correct” model. HAC errors are a remedy. It´s hard to understand. When we use this to estimate “robust” standard errors for our coefficients we get slightly different estimates. A point in the upper or lower right corners is an observation exhibiting influence on the model. (Though admittedly, the loss of power in this simulation is rather small.). We would use the vcovHC function in the sandwich package as we demonstrated at the beginning of this post along with the coeftest function from the lmtest package. After clicking on the OK button, the output from the data analysis tool is shown on the right side of Figure 2. However, it seems JavaScript is either disabled or not supported by your browser. If you send me an Excel file with your data and regression analysis, I can try to figure out what is going on. Now let’s take a closer look at the “meat” in this sandwich formula: That is a matrix of constant variance. We discuss the motivation for a modification suggested by Bell and McCaffrey (2002) to improve the finite sample properties of the confidence intervals based on the conventional robust standard errors. Our 5th observation has a corner all to itself. “Econometric Computing with HC and HAC Covariance Matrix Estimators.”, Zeileis A (2006). 2. It might not surprise you there are several ways. Learn more about robust standard errors, linear regression, robust linear regression, robust regression, linearmodel.fit Statistics and Machine Learning Toolbox, Econometrics Toolbox To make this easier to demonstrate, we’ll use a small toy data set. That is why the standard errors are so important: they are crucial in determining how many stars your table gets. For example, the range H17:I20 contains the worksheet array formula =RRegCoeff(C4:E53,B4:B53. Related to this last point, Freedman (2006) expresses skepticism about even using robust standard errors: If the model is nearly correct, so are the usual standard errors, and robustification is unlikely to help much. Second, if the model is not correctly specified, the sandwich estimators are only useful if the parameters estimates are still consistent, i.e., if the misspecification does not result in bias. Of course we wouldn’t typically calculate robust standard errors by hand like this. It is simply the number 5 with some random noise from a N(0,1.2) distribution plus the number 35. We may be missing key predictors, interactions, or non-linear effects. “On the So-called ‘Huber Sandwich Estimator’ and ‘Robust Standard Errors’.” Lecture Notes. Even when the homogeneity of variance assumption is violated the ordinary least squares (OLS) method calculates unbiased, consistent estimates of the population regression coefficients. Before we do that, let’s use this formula by hand to see how it works when we calculate the usual standard errors. Fill in the dialog box that appears as shown in Figure 1. (Or use vce(hc3) in Stata). The newer GENLINMIXED procedure (Analyze>Mixed Models>Generalized Linear) offers similar capabilities. This is the idea of “robust” standard errors: modifying the “meat” in the sandwich formula to allow for things like non-constant variance (and/or autocorrelation, a phenomenon we don’t address in this post). Real Statistics Data Analysis Tool: The Multiple Linear Regression data analysis tool contains an option for calculating any one of the versions of the Huber-White’s Robust Standard Errors described above. URL, Zeileis A (2004). For example, it might make sense to assume the error of the 5th data point was drawn from a Normal distribution with a larger variance. The standard errors determine how accurate is your estimation. The formula for “HC1” is as follows: where \(\hat{\mu}_i^2\) refers to squared residuals, \(n\) is the number of observations, and \(k\) is the number of coefficients. Let’s modify our formula above to substitute HC1 “meat” in our sandwich: Notice we no longer have constant variance for each observation. E[e] = 0 and E[eeT] = 0, means that S is the diagonal matrix whose diagonal elements are . However, when we regress y on x using lm we get a slope coefficient of about 5.2 that appears to be “significant”. \[\text{Var}(\hat{\beta}) = (X^TX)^{-1} X^T\Omega X (X^TX)^{-1}\], http://www.stat.berkeley.edu/~census/mlesan.pdf, Visit the Status Dashboard for at-a-glance information about Library services, Freedman DA (2006). Those are the kinds of questions this post intends to address. If you use robust standard errors, then the results should be pretty good. But note that inference using these standard errors is only valid for sufficiently large sample sizes (asymptotically normally distributed t-tests). Hello, I tried to run multi-variable regression per your instruction using the regression add-in provided, but it only gives me same results as non-robust standard error tests – why is that? 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