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Regress Postestimation — Postestimation Tools for Regress Title stata.com regress postestimation — Postestimation tools for regress Description Predictions DFBETA influence statistics Tests for violation of assumptions Variance inflation factors Measures of effect size Methods and formulas Acknowledgments References Also see Description The following postestimation commands are of special interest after regress: Command Description dfbeta DFBETA influence statistics estat hettest tests for heteroskedasticity estat imtest information matrix test estat ovtest Ramsey regression specification-error test for omitted variables estat szroeter Szroeter’s rank test for heteroskedasticity estat vif variance inflation factors for the independent variables estat esize η2 and !2 effect sizes These commands are not appropriate after the svy prefix. 1 2 regress postestimation — Postestimation tools for regress The following standard postestimation commands are also available: Command Description contrast contrasts and ANOVA-style joint tests of estimates estat ic Akaike’s and Schwarz’s Bayesian information criteria (AIC and BIC) estat summarize summary statistics for the estimation sample estat vce variance–covariance matrix of the estimators (VCE) estat (svy) postestimation statistics for survey data estimates cataloging estimation results forecast1 dynamic forecasts and simulations hausman Hausman’s specification test lincom point estimates, standard errors, testing, and inference for linear combinations of coefficients linktest link test for model specification lrtest2 likelihood-ratio test margins marginal means, predictive margins, marginal effects, and average marginal effects marginsplot graph the results from margins (profile plots, interaction plots, etc.) nlcom point estimates, standard errors, testing, and inference for nonlinear combinations of coefficients predict predictions, residuals, influence statistics, and other diagnostic measures predictnl point estimates, standard errors, testing, and inference for generalized predictions pwcompare pairwise comparisons of estimates suest seemingly unrelated estimation test Wald tests of simple and composite linear hypotheses testnl Wald tests of nonlinear hypotheses 1 forecast is not appropriate with mi or svy estimation results. 2 lrtest is not appropriate with svy estimation results. regress postestimation — Postestimation tools for regress 3 Predictions Syntax for predict predict type newvar if in , statistic statistic Description Main xb linear prediction; the default residuals residuals score score; equivalent to residuals rstandard standardized residuals rstudent Studentized (jackknifed) residuals cooksd Cook’s distance leverage j hat leverage (diagonal elements of hat matrix) pr(a,b) Pr(yj j a < yj < b) e(a,b) E(yj j a < yj < b) ∗ ∗ ystar(a,b) E(yj ), yj = maxfa; min(yj; b)g ∗ dfbeta(varname) DFBETA for varname stdp standard error of the linear prediction stdf standard error of the forecast stdr standard error of the residual ∗ covratio COVRATIO ∗ dfits DFITS ∗ welsch Welsch distance Unstarred statistics are available both in and out of sample; type predict ::: if e(sample) ::: if wanted only for the estimation sample. Starred statistics are calculated only for the estimation sample, even when if e(sample) is not specified. rstandard, rstudent, cooksd, leverage, dfbeta(), stdf, stdr, covratio, dfits, and welsch are not available if any vce() other than vce(ols) was specified with regress. xb, residuals, score, and stdp are the only options allowed with svy estimation results. where a and b may be numbers or variables; a missing (a ≥ :) means −1, and b missing (b ≥ :) means +1; see [U] 12.2.1 Missing values. Menu for predict Statistics > Postestimation > Predictions, residuals, etc. Options for predict Main £ £xb, the default, calculates the linear prediction. residuals calculates the residuals. score is equivalent to residuals in linear regression. 4 regress postestimation — Postestimation tools for regress rstandard calculates the standardized residuals. rstudent calculates the Studentized (jackknifed) residuals. cooksd calculates the Cook’s D influence statistic (Cook 1977). leverage or hat calculates the diagonal elements of the projection (“hat”) matrix. pr(a,b) calculates Pr(a < xjb + uj < b), the probability that yjjxj would be observed in the interval (a; b). a and b may be specified as numbers or variable names; lb and ub are variable names; pr(20,30) calculates Pr(20 < xjb + uj < 30); pr(lb,ub) calculates Pr(lb < xjb + uj < ub); and pr(20,ub) calculates Pr(20 < xjb + uj < ub). a missing (a ≥ :) means −∞; pr(.,30) calculates Pr(−∞ < xjb + uj < 30); pr(lb,30) calculates Pr(−∞ < xjb + uj < 30) in observations for which lb ≥ : and calculates Pr(lb < xjb + uj < 30) elsewhere. b missing (b ≥ :) means +1; pr(20,.) calculates Pr(+1 > xjb + uj > 20); pr(20,ub) calculates Pr(+1 > xjb + uj > 20) in observations for which ub ≥ : and calculates Pr(20 < xjb + uj < ub) elsewhere. e(a,b) calculates E(xjb + uj j a < xjb + uj < b), the expected value of yjjxj conditional on yjjxj being in the interval (a; b), meaning that yjjxj is truncated. a and b are specified as they are for pr(). ∗ ∗ ∗ ystar(a,b) calculates E(yj ), where yj = a if xjb + uj ≤ a, yj = b if xjb + uj ≥ b, and ∗ ∗ yj = xjb + uj otherwise, meaning that yj is censored. a and b are specified as they are for pr(). dfbeta(varname) calculates the DFBETA for varname, the difference between the regression coefficient when the jth observation is included and excluded, said difference being scaled by the estimated standard error of the coefficient. varname must have been included among the regressors in the previously fitted model. The calculation is automatically restricted to the estimation subsample. stdp calculates the standard error of the prediction, which can be thought of as the standard error of the predicted expected value or mean for the observation’s covariate pattern. The standard error of the prediction is also referred to as the standard error of the fitted value. stdf calculates the standard error of the forecast, which is the standard error of the point prediction for 1 observation. It is commonly referred to as the standard error of the future or forecast value. By construction, the standard errors produced by stdf are always larger than those produced by stdp; see Methods and formulas. stdr calculates the standard error of the residuals. covratio calculates COVRATIO (Belsley, Kuh, and Welsch 1980), a measure of the influence of the jth observation based on considering the effect on the variance–covariance matrix of the estimates. The calculation is automatically restricted to the estimation subsample. dfits calculates DFITS (Welsch and Kuh 1977) and attempts to summarize the information in the leverage versus residual-squared plot into one statistic. The calculation is automatically restricted to the estimation subsample. welsch calculates Welsch distance (Welsch 1982) and is a variation on dfits. The calculation is automatically restricted to the estimation subsample. regress postestimation — Postestimation tools for regress 5 Remarks and examples for predict Remarks are presented under the following headings: Terminology Fitted values and residuals Prediction standard errors Prediction with weighted data Leverage statistics Standardized and Studentized residuals DFITS, Cook’s Distance, and Welsch Distance COVRATIO Terminology Many of these commands concern identifying influential data in linear regression. This is, un- fortunately, a field that is dominated by jargon, codified and partially begun by Belsley, Kuh, and Welsch(1980). In the words of Chatterjee and Hadi(1986, 416), “Belsley, Kuh, and Welsch’s book, Regression Diagnostics, was a very valuable contribution to the statistical literature, but it unleashed on an unsuspecting statistical community a computer speak (a` la Orwell), the likes of which we have never seen.” Things have only gotten worse since then. Chatterjee and Hadi’s (1986, 1988) own attempts to clean up the jargon did not improve matters (see Hoaglin and Kempthorne[1986], Velleman[1986], and Welsch[1986]). We apologize for the jargon, and for our contribution to the jargon in the form of inelegant command names, we apologize most of all. Model sensitivity refers to how estimates are affected by subsets of our data. Imagine data on y and x, and assume that the data are to be fit by the regression yi = α + βxi + i. The regression estimates of α and β are a and b, respectively. Now imagine that the estimated a and b would be different if a small portion of the dataset, perhaps even one observation, were deleted. As a data analyst, you would like to think that you are summarizing tendencies that apply to all the data, but you have just been told that the model you fit is unduly influenced by one point or just a few points and that, as a matter of fact, there is another model that applies to the rest of the data—a model that you have ignored. The search for subsets of the data that, if deleted, would change the results markedly is a predominant theme of this entry. There are three key issues in identifying model sensitivity to individual observations, which go by the names residuals, leverage, and influence. In our yi = a + bxi + ei regression, the residuals are, of course, ei —they reveal how much our fitted value ybi = a + bxi differs from the observed yi.A point (xi; yi) with a corresponding large residual is called an outlier. Say that you are interested in outliers because you somehow think that such points will exert undue influence on your estimates. Your feelings are generally right, but there are exceptions. A point might have a huge residual and yet not affect the estimated b at all. Nevertheless, studying observations with large residuals almost always pays off. (xi; yi) can be an outlier in another way—just as yi can be far from ybi, xi can be far from the center of mass of the other x’s. Such an “outlier” should interest you just as much as the more traditional outliers. Picture a scatterplot of y against x with thousands of points in some sort of mass at the lower left of the graph and one point at the upper right of the graph.
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