glmtoolbox (version 0.1.12)

spruces: Effect of ozone-enriched atmosphere on growth of sitka spruces

Description

These data are analyzed primarily to determine how ozone pollution affects tree growth. As ozone pollution is common in urban areas, the impact of increased ozone concentrations on tree growth is of considerable interest. The response variable is tree size, where size is conventionally measured by the product of tree height and stem diameter squared. In the first group, 54 trees were grown under an ozone-enriched atmosphere, ozone exposure at 70 parts per billion. In the second group, 25 trees were grown under normal conditions. The size of each tree was observed 13 times across time, that is, 152, 174, 201, 227, 258, 469, 496, 528, 556, 579, 613, 639 and 674 days since the beginning of the experiment. Hence, the objective is to compare the trees' growth patterns under the two conditions.

Usage

data(spruces)

Format

A data frame with 1027 rows and 4 variables:

tree

a factor giving an unique identifier for each tree.

days

a numeric vector giving the number of days since the beginning of the experiment.

size

a numeric vector giving an estimate of the volume of the tree trunk.

treat

a factor giving the treatment received for each tree: "normal" and "ozone-enriched".

References

Diggle P.J., Heagarty P., Liang K.-Y., Zeger S.L. (2002) Analysis of Longitudinal Data. Oxford University Press, Oxford.

Crainiceanu C.M., Ruppert D., Wand M.P. (2005). Bayesian Analysis for Penalized Spline Regression Using WinBUGS. Journal of Statistical Software 14(14):1-24.

Examples

data(spruces)
dev.new()
boxplot(size ~ days, data=subset(spruces,treat=="normal"), at=c(1:13) - 0.2,
        col="yellow", boxwex=0.3, xaxt="n", xlim=c(0.9,13.1))
boxplot(size ~ days, data=subset(spruces,treat=="ozone-enriched"), add=TRUE,
        at=c(1:13) + 0.2, col="blue", boxwex=0.3, xaxt="n")
axis(1, at=c(1:13), labels=unique(spruces$days))
axis(2, at=seq(0,2000,250), labels=seq(0,2000,250))
legend("topleft", legend=c("normal","ozone-enriched"), fill=c("yellow","blue"),
       title="Atmosphere", bty="n")