cen2way {NADA2}

Parametric Two Factor Fixed Effects ANOVA for censored data

Description

Computes tests for each of the two factors and optionally for their interaction using likelihood ratio tests. p-values will not be identical to the usual method of moments ANOVA tests but will be similar.

Usage

cen2way(y1, y2, fac1, fac2, LOG = TRUE, interact = TRUE)

Arguments

y1

The column of data values plus detection limits

y2

The column of indicators, where 1 (or TRUE) indicates a detection limit in the y1 column, and 0 (or FALSE) indicates a detected value in y1.

fac1

The first grouping or factor variable. Can be either a text or numeric value indicating the group assignment.

fac2

The second grouping or factor variable. Can be either a text or numeric value indicating the group assignment.

LOG

A logical variable indicating whether natural logs are to be taken of the 'y1' column data. Default is TRUE.

interact

A logical variable indicating whether to compute an interaction term between the two variables. Default is TRUE. #' @keywords two-way two-factor factorial ANOVA analysis of variance censored

Details

Tests are computed using Maximum Likelihood Estimation. When a gaussian distribution model is used (LOG=FALSE) modeled values may fall below zero, producing unreal p-values (often lower than they should be). Because of this, testing in log units is preferable and is the default unless you are transforming the y values prior to running the function (such as taking cube roots to approximate a gamma distribution). The 'delta.lr0x2' stat output is the -2loglikehood for the test of the model versus an intercept-only model.

Value

Q-Q plots of residuals. Likelihood ratio test statistics ("chisquare"), degrees of freedom ("df") and p-values (pval) for two factors and optionally the interaction. Data on the underlying models, including AIC and R2 are also provided.

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.

See Also

survival::survreg

Examples

data(Gales_Creek)
Gales_Creek$Period <- c(rep("early", 35), rep("middle", 12), rep("late", 16))
with(Gales_Creek,cen2way(TCr, CrND, Season, Period))