quantreg (version 3.52)

anova.rq: Anova function for quantile regression fits

Description

Compute test statistics for two or more quantile regression fits.

Usage

anova.rq(object, ...)
anova.rqlist(object, ...,  test = "Wald", score = "tau")

Arguments

object, ...

objects of class `rq', originating from a call to `rq'.

test

A character string specifying the test statistic to use. Can be either `Wald' or `rank'.

score

A character string specifying the score function to use, only needed or applicable for the `rank' form of the test.

Value

• An object of class `"anova"' inheriting from class `"data.frame"'.

WARNING

An attempt to verify that the models are nested in the first form of the test is made, but this relies on checking set inclusion of the list of variable names and is subject to obvious ambiguities when variable names are generic. The comparison between two or more models will only be valid if they are fitted to the same dataset. This may be a problem if there are missing values and R's default of `na.action = na.omit' is used. The rank version of the nested model tests involves computing the entire regression quantile process using parametric linear programming and thus can be rather slow and memory intensive on problems with more than several thousand observations.

Details

There are two (as yet) distinct forms of the test. In the first the fitted objects all have the same specified quantile (tau) and the intent is to test the hypothesis that smaller models are adaquete relative to the largest specified model. In this case there are two options for the argument `test', by default a Wald test is computed as in Bassett and Koenker (1982). If test=`rank' is specified, then a rank test statistic is computed as described in Gutenbrunner, Jureckova, Koenker and Portnoy (1993). In the latter case one can also specify a form for the score function of the rank test, by default the Wilcoxon score is used, the other options are score=`sign' for median (sign) scores, or score=`normal' for normal (van der Waerden) scores. A fourth option is score=`tau' which is a generalization of median scores to an arbitrary quantile, in this case the quantile is assumed to be the one associated with the fitting of the specified objects. The computing of the rank form of the test is carried out in the rq.test.rank function, see ranks for further details on the score function options.

The Wald form of the test is local in sense that the null hypothesis asserts only that a subset of the covariates are ``insignificant'' at the specified quantile of interest. The rank form of the test can also be used to test the global hypothesis that a subset is ``insignificant'' over an entire range of quantiles. The use of the score function score = "tau" restricts the rank test to the local hypothesis of the Wald test.

In the second form of the test the linear predictor of the fits are all the same, but the specified quantiles (taus) are different. In this case the hypothesis of interest is that the slope coefficients of the models are identical. The test statistic is a variant of the Wald test described in Koenker and Bassett (1982).

By default, the tests return an F-like statistic in the sense that the an asymptotically Chi-squared statistic is divided by its degrees of freedom and the reported p-value is computed for an F statistic based on the numerator degrees of freedom equal to the rank of the null hypothesis and the denominator degrees of freedom is taken to be the sample size minus the number of parameters of the maintained model.

References

[1] Bassett, G. and R. Koenker (1982). Tests of Linear Hypotheses and L1 Estimation, Econometrica, 50, 1577--83.

[2] Koenker, R. W. and Bassett, G. W. (1982). Robust Tests for Heteroscedasticity based on Regression Quantiles, Econometrica, 50, 43--61. [3] Gutenbrunner, C., Jureckova, J., Koenker, R, and S. Portnoy (1993). Tests of Linear Hypotheses based on Regression Rank Scores, J. of Nonparametric Statistics, 2, 307--331.

See Also

The model fitting function rq, and the functions for testing hypothesis on the entire quantile regression process khmaladze.test. For further details on the rank tests see ranks.

Examples

data(barro)
fit0 <- rq(y.net ~  lgdp2 + fse2 + gedy2 , data = barro)
fit1 <- rq(y.net ~  lgdp2 + fse2 + gedy2 + Iy2 + gcony2, data = barro)
fit2 <- rq(y.net ~  lgdp2 + fse2 + gedy2 + Iy2 + gcony2, data = barro,tau=.75)
fit3 <- rq(y.net ~  lgdp2 + fse2 + gedy2 + Iy2 + gcony2, data = barro,tau=.25)
anova(fit1,fit0)
anova(fit1,fit2,fit3)