drc (version 3.0-1)

lettuce: Hormesis in lettuce plants

Description

Data are from an experiment where isobutylalcohol was dissolved in a nutrient solution in which lettuce (Lactuca sativa) plants were grown. The plant biomass of the shoot was determined af 21 days.

Usage

data(lettuce)

Format

A data frame with 14 observations on the following 2 variables.

Source

van Ewijk, P. H. and Hoekstra, J. A. (1993) Calculation of the EC50 and its Confidence Interval When Subtoxic Stimulus Is Present, ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY, 25, 25--32.

Details

The data set illustrates hormesis, presence of a subtoxic stimulus at low concentrations.

References

van Ewijk, P. H. and Hoekstra, J. A. (1994) Curvature Measures and Confidence Intervals for the Linear Logistic Model, Appl. Statist., 43, 477--487.

Examples

## Look at data
lettuce

## Monotonous dose-response model
lettuce.m1 <- drm(weight~conc, data=lettuce, fct=LL.3())

plot(lettuce.m1, broken = TRUE)

## Model fit in van Ewijk and Hoekstra (1994)
lettuce.m2 <- drm(weight~conc, data=lettuce, fct=BC.4())
modelFit(lettuce.m2)

plot(lettuce.m2, add = TRUE, broken = TRUE, type = "none", lty = 2)

## Hormesis effect only slightly significant
summary(lettuce.m2)

## Hormesis effect highly significant
##  compare with t-test for the "f" parameter in the summary output)
anova(lettuce.m1, lettuce.m2)