drink <- read.table( "drink.dat", header = T )
drink2 <- drink[drink$period < 3,]
drink4 <- drink[!is.na(match(drink$cow,drink$cow[drink$period == 4])),]

drink$cow <- factor( drink$cow )
drink$month <- ordered( drink$month )
drink$period <- ordered( drink$period )
drink.fit <- aov( milk ~ cow + period * water, drink )

drink2$cow <- factor( drink2$cow )
drink2$month <- ordered( drink2$month )
drink2$period <- ordered( drink2$period )
drink2.fit <- aov( milk ~ cow + period * water, drink2 )

drink4$cow <- factor( drink4$cow )
drink4$month <- ordered( drink4$month )
drink4$period <- ordered( drink4$period )
drink4.fit <- aov( milk ~ cow + period * water, drink4 )