agridat (version 1.8.1)

streibig.competition: Competition experiment between barley and sinapis.

Description

Competition experiment between barley and sinapis, at different planting rates.

source

Peter McCullagh, John A. Nelder. Generalized Linear Models, page 318-320. Used with permission of Jens Streibig.

Details

The source data (in McCullagh) also contains a count of plants harvested (not included here) that sometimes is greater than the number of seeds planted.

References

Oliver Schabenberger and Francis J Pierce. 2002. Contemporary Statistical Models for the Plant and Soil Sciences, CRC Press. Page 370-375.

Examples

dat <- streibig.competition

# See Schaberger and Pierce, pages 370+
# Consider only the mono-species barley data (no competition from sinapis)
d1 <- subset(dat, sseeds<1)
d1 <- transform(d1, x=bseeds, y=bdwt, block=factor(block))

# Inverse yield looks like it will be a good fit for Gamma's inverse link
xyplot(1/y~x, data=d1, group=block, auto.key=TRUE,
       xlab="Seeding rate", ylab="Inverse yield of barley dry weight")

# linear predictor is quadratic, with separate intercept and slope per block
m1 <- glm(y ~ block + block:x + x+I(x^2), data=d1,
          family=Gamma(link="inverse"))
# Predict and plot
newdf <- expand.grid(x=seq(0,120,length=50), block=factor(c('B1','B2','B3')) )
newdf$pred <- predict(m1, new=newdf, type='response')
plot(y~x, data=d1, col=block)
for(bb in 1:3){
  newbb <- subset(newdf, block==c('B1','B2','B3')[bb])
  lines(pred~x, data=newbb, col=bb)
}