cenanova: ANOVA for censored data In NADA2: Data Analysis for Censored Environmental Data

Description

Performs a parametric test of differences in means between groups of censored data, followed by a parametric Tukey's multiple comparison test.

Usage

 1 cenanova(y1, y2, grp, LOG = TRUE, printstat = 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. grp Grouping or factor variable. Can be either a text or numeric value indicating the group assignment. LOG Indicator of whether to compute tests in the original 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). printstat Logical TRUE/FALSE option of whether to print the resulting statistics in the console window, or not. Default is TRUE.

Details

Test is 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.

Value

Returns the Maximum Likelihood Estimation (MLE) comparison results including Chi-Squared value, degrees of freedom and p-value of the test. Test assumes log-normal(LOG=TRUE) or normal(LOG=FALSE) distribution of residuals from group means.

Tukey's multiple comparison p-values of pairwise differences in group means are also printed.

• Group Names of groups (NOTE: == 0 indicates null hypothesis of "equals zero").

• Estimate Estimated difference between group means.

• Std. Error Standard error of estimate.

• z value Test statistic.

• Pr(>|z|) P-values for test that difference in means equals zero.

References

Helsel, D.R., 2011. Statistics for Censored Environmental Data using Minitab and R, 2nd ed. John Wiley & Sons, USA, N.J.

#' @examples data(PbHeron) cenanova(PbHeron\$Liver,PbHeron\$LiverCen,PbHeron\$DosageGroup)

cenanova(PbHeron\$Liver,PbHeron\$LiverCen,PbHeron\$DosageGroup,LOG=FALSE)