check.qgam {qgam}

Some diagnostics for a fitted qgam model

Description

Takes a fitted gam object produced by qgam() and produces some diagnostic information about the fitting procedure and results. It is partially based on mgcv::gam.check.

Usage

## S3 method for class 'qgam'
check(obj, nbin = 10, lev = 0.05, ...)

Arguments

obj

the output of a qgam() call.

nbin

number of bins used in the internal call to cqcheck().

lev

the significance levels used by cqcheck(), which determines the width of the confidence intervals.

...

extra arguments to be passed to plot()

Details

This function provides two plots. The first shows how the number of responses falling below the fitted quantile (y-axis) changes with the fitted quantile (x-axis). To be clear: if the quantile is fixed to, say, 0.5 we expect 50% of the responses to fall below the fit. See ?cqcheck() for details. The second plot related to |F(hat(mu)) - F(mu0)|, which is the absolute bias attributable to the fact that qgam is using a smoothed version of the pinball-loss. The absolute bias is evaluated at each observation, and an histogram is produced. See Fasiolo et al. (2017) for details. The function also prints out the integrated absolute bias, and the proportion of observations lying below the regression line. It also provides some convergence diagnostics (regarding the optimization), which are the same as in mgcv::gam.check. It reports also the maximum (k') and the selected degrees of freedom of each smooth term.

Value

Simply produces some plots and prints out some diagnostics.

Author(s)

Matteo Fasiolo <[e-mail]>, Simon N. Wood.

References

Fasiolo, M., Wood, S.N., Zaffran, M., Nedellec, R. and Goude, Y., 2020. Fast calibrated additive quantile regression. Journal of the American Statistical Association (to appear). https://www.tandfonline.com/doi/full/10.1080/01621459.2020.1725521.

Examples

library(qgam)
set.seed(0)
dat <- gamSim(1, n=200)
b<-qgam(y~s(x0)+s(x1)+s(x2)+s(x3), data=dat, qu = 0.5)
plot(b, pages=1)
check.qgam(b, pch=19, cex=.3)