mgcViz (version 0.2.0)

l_gridQCheck1D: Checking sign of residuals along one covariate

Description

This layer is mainly useful when checking quantile GAMs fitted using the qgam package. The residuals, r, are binned according to the corresponding value of a covariate, x. Then the proportions of negative residuals within each bin are calculated, and compared with the theoretical value, qu. Confidence intervals for the proportion of negative residuals can be derived using binomial quantiles (under an independence assumption). To be used in conjuction with check1D.

Usage

l_gridQCheck1D(qu = NULL, n = 20, level = 0.8, ...)

Value

An object of class gamLayer

Arguments

qu

the quantile of interest. Should be in (0, 1).

n

number of grid intervals.

level

the level of the confidence intervals plotted.

...

graphical arguments to be passed to ggplot2::geom_point.

Examples

# Simulate some data
library(mgcViz)
set.seed(3841)
dat <- gamSim(1,n=400,dist="normal",scale=2)
dat$fac <- as.factor( sample(letters[1:8], nrow(dat), replace = TRUE) ) 
fit <- qgam(y~s(x1)+s(x2)+s(x3)+fac, data=dat, err = 0.05, qu = 0.4)
fit <- getViz(fit)

# "x0" effect is missing, but should be there. l_gridQCheck1D shows
# that fraction of negative residuals is quite different from the theoretical 0.4
# in several places along "x0".
check1D(fit, dat$x0) + l_gridQCheck1D(qu = 0.4, n = 20)
# The problem gets better if s(x0) is added to the model.

# Works also with factor variables
check1D(fit, "fac") + l_gridQCheck1D(qu = 0.4)