data <- scan(what = list(group="",age=0,change=0), multi.line=F)
 aerobic 31 17.05   aerobic 23 4.96   aerobic 27 10.40 
 aerobic 28 11.05   aerobic 22 0.26   aerobic 24 2.51 
 running 23 -0.87   running 22 -10.74  running 22 -3.27 
 running 25 -1.97   running 27 7.50   running 20 -7.25 

data
     
table171 <- data.frame(data)
rm(data)

table171$group  <- as.factor(table171$group)


 # look at the data

plot(table171$age,table171$change,pch=as.numeric(table171$group),
      col=as.numeric(table171$group),ylab="Change",xlab="Age")
legend("bottomright",levels(table171$group),pch=1:2,col=1:2)


 # standard ANCOVA assuming equal slopes

tab171.lm1 <- lm(change ~ age +group , data=table171)


 # check assumptions

windows()
par(mfrow=c(2,2))  
plot(tab171.lm1)
par(mfrow=c(1,1))  



 # check equal slopes assumption

tab171.lm2 <- lm(change ~ age +group +age:group , data=table171)