## 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.

- conc
a numeric vector of concentrations of isobutylalcohol (mg/l)

- weight
a numeric vector of biomass of shoot (g)

### Details

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

### 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.

### 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)