Q-Q plot of censored regression residuals

Plots a quantile-quantile (Q-Q) plot of censored regression residuals for simple or multiple regression.

cenregQQ(y.var, cen.var, x.vars, LOG = TRUE, intcens = FALSE, main = NULL)

Arguments

y.var

The column of y (response variable) values plus detection limits. Alternatively, with interval-censord data, the column of the lower end of the interval.

cen.var

The column of indicators, where 1 (or TRUE) indicates a detection limit in the y.var column, and 0 (or FALSE) indicates a detected value in y.var. Alternatively, with interval-censored data the column of the high end of the interval.

x.vars

One or more uncensored explanatory variable(s). See Details

LOG

Indicator of whether to compute the regression in the original y units, or on their logarithms. The default is to use the logarithms (LOG = TRUE). To compute in original units, specify the option LOG = FALSE (or LOG = 0).

intcens

a logical value indicating the input data is interval-censored instead of a column of values plus a column of indicators.

main

overall title for the plot. A default titel will be used if none is specified.

Value

Q-Q Plot of model residuals and Shapiro-Francia test results.

References

Helsel, D.R., 2011. Statistics for censored environmental data using Minitab and R, 2nd ed. John Wiley & Sons, USA, N.J.

Millard, S.P., 2013. EnvStats: An R Package for Environmental Statistics. Springer-Verlag, New York.

Shapiro, S.S., Francia, R.S., 1972. An approximate analysis of variance test for normality. Journal of the American Statistical Association 67, 215–216.

Examples

data(Brumbaugh)

# One variable
cenregQQ(Brumbaugh$Hg,Brumbaugh$HgCen,Brumbaugh$PctWetland)


# More than one variable for demostration purposes
cenregQQ(Brumbaugh$Hg,Brumbaugh$HgCen,Brumbaugh[,c("PctWetland","SedLOI","Weight")])