### A function for Mallow's CP statistic.

### This function takes 2 arguments; 
### 1. A saved "lm" object from the model of interest.
### 2. A saved "lm" object from the kitchen sink model

mallows <- function(small.mod,large.mod)
{
### number of parameters in the small model
num.params <- small.mod$rank

### MSE for the small model
s.sq <- (summary(small.mod)$sigma)^2

### MSE for kitchen sink
sig.sq <- (summary(large.mod)$sigma)^2

### residual df for kitchen sink model
res.df <- summary(small.mod)$df[2]

mallowscp <- num.params + (s.sq - sig.sq) * res.df / sig.sq

return(mallowscp) 

}

### Calculating the press residuals for an lm object

press.resid <- function(my.model)
{ 

press.resids <-  residuals(my.model)/(1 - lm.influence(my.model)$hat)

press.stat <- sum(press.resids^2)

return(press.resids)

}

### Calculating the press statistic for an lm object

press.stat <- function(my.model)
{
return(sum(press.resid(my.model)^2))
}