1. Q1:
Ans:
data <- read.table("http://fisher.stat.wmich.edu/joe/Stat2600/GA/xmaty.text",sep="
",header=TRUE)
attach(data)
head(data)
y <- data[,7]
xmat <- data[,1:6]
> head(data)
x1 x2 x3 x4 x5 x6 y
1 38.7 13.5 70.2 62.6 60.8 58.6 -12.7
2 69.0 75.2 48.7 43.2 68.6 64.7 -36.4
3 55.1 76.5 51.5 67.5 58.5 87.9 -53.9
4 77.3 84.7 48.5 79.5 74.2 93.9 -56.5
5 62.4 27.3 112.8 63.0 55.9 73.9 -83.8
6 49.5 47.8 42.9 79.4 53.0 83.7 -20.2
Q2:
Ans:
lmbest2 <- function(y,xmat){
# This returns R-squared and sigmahat for all linear
# models of size 2 (2 x's). The results are sorted on R-squared.
p <- length(xmat[1,])
coll <- c()
for(i in 1:(p-1)){
for(j in (i+1):p){
fit <- lm(y ~ xmat[,i] + xmat[,j])
sfit <- summary(fit)
tmp <- c(i,j,sfit$sigma,sfit$r.squared)
coll <- rbind(coll,tmp)
}
}
ind <- order(coll[,4],decreasing=TRUE)
collf <- coll[ind,]
colnames(collf) <- c("i","j","sigmahat","R-squared")
ic <- length(collf[,1])

2. rownames(collf) <- rep("Model",ic)
return(collf)
}
Q3:
Ans:
lmbest2(y,xmat)
Top 3 models are:
For better model we need lower sigmahat and higher R-squared. Model 1 has lowest sigmahat
and highest R-squared followed by Model 2 then model 3.
Hence among these three Model 1 is the best fit and Model 3 is the worst fit and model 2 is in
between the other two models.
From the value of R-squared = 0.479 we can say that there are other models which are better fit
because for good fits R-squared should be close to 1. Hence I would like to choose another
models with more variables involved.
Solution
Q1:
Ans:
data <- read.table("http://fisher.stat.wmich.edu/joe/Stat2600/GA/xmaty.text",sep="
",header=TRUE)
attach(data)
head(data)
y <- data[,7]
xmat <- data[,1:6]
> head(data)
x1 x2 x3 x4 x5 x6 y
1 38.7 13.5 70.2 62.6 60.8 58.6 -12.7
2 69.0 75.2 48.7 43.2 68.6 64.7 -36.4
3 55.1 76.5 51.5 67.5 58.5 87.9 -53.9
4 77.3 84.7 48.5 79.5 74.2 93.9 -56.5
5 62.4 27.3 112.8 63.0 55.9 73.9 -83.8
6 49.5 47.8 42.9 79.4 53.0 83.7 -20.2
Q2:
Ans:
lmbest2 <- function(y,xmat){

3. # This returns R-squared and sigmahat for all linear
# models of size 2 (2 x's). The results are sorted on R-squared.
p <- length(xmat[1,])
coll <- c()
for(i in 1:(p-1)){
for(j in (i+1):p){
fit <- lm(y ~ xmat[,i] + xmat[,j])
sfit <- summary(fit)
tmp <- c(i,j,sfit$sigma,sfit$r.squared)
coll <- rbind(coll,tmp)
}
}
ind <- order(coll[,4],decreasing=TRUE)
collf <- coll[ind,]
colnames(collf) <- c("i","j","sigmahat","R-squared")
ic <- length(collf[,1])
rownames(collf) <- rep("Model",ic)
return(collf)
}
Q3:
Ans:
lmbest2(y,xmat)
Top 3 models are:
For better model we need lower sigmahat and higher R-squared. Model 1 has lowest sigmahat
and highest R-squared followed by Model 2 then model 3.
Hence among these three Model 1 is the best fit and Model 3 is the worst fit and model 2 is in
between the other two models.
From the value of R-squared = 0.479 we can say that there are other models which are better fit
because for good fits R-squared should be close to 1. Hence I would like to choose another
models with more variables involved.