file <- "http://www.uvm.edu/~rsingle/stat211/data/extra/ch6_BatteryLife.txt"
 dat<-read.table(file, header=T, na.strings=c("NA",".")) 

#split the screen
# par(mfrow=c(2,2))

#check assumption of equal variances 
 boxplot(lifetime ~ battery, data=dat)
 var.test(lifetime~battery, data=dat) #does NOT work for >2 groups
 by(dat$lifetime, dat$battery, var)   #by(DV, IV, FUNCTION) for Fmax test
 aggregate(lifetime ~ battery, data=dat, FUN=function(x){c(Mean=mean(x), SD=sd(x))})
 bartlett.test(lifetime~battery,data=dat)

 
#ANOVA model
 mod.aov <- aov(lifetime ~ as.factor(battery), data=dat)
 mod.aov
 hist(mod.aov$resid, nclass=)
 summary(mod.aov)
	 
	#WRONG - regression model treating battery as continuous
	 Xmod.aov <- aov(lifetime ~ battery, data=dat)
	 summary(Xmod.aov)




#assign local variables accessible without dat$ prefix
 battery <- dat$battery
 lifetime <- dat$lifetime

 t.test(lifetime[battery!=3] ~ battery[battery!=3], var.equal=TRUE)
 t.test(lifetime[battery!=2] ~ battery[battery!=2], var.equal=TRUE)
 t.test(lifetime[battery!=1] ~ battery[battery!=1], var.equal=TRUE)

 pairwise.t.test(lifetime, battery, p.adj = "none")
 
 aggregate(lifetime ~ battery, data=dat, FUN=function(x){c(Mean=mean(x), SD=sd(x))})