twosamp <- function(yvec,trtvec,alpha=0.05,header="") {
#################################################
# A function to compute a two-sample t-test and confidence 
# interval (equal-variance, independent samples).  yvec is 
# a numeric vector containing both samples' data.  trtvec 
# is a vector, same length as yvec, of treatment
# identifiers for the data in yvec.  A boxplot comparing
# the treatments' data is constructed.  Output is a one-row
# data frame reporting the results of the test and 
# confidence interval
##################################################
trtvec<-as.factor(trtvec)
boxplot(split(yvec,trtvec))
title(header)
ybar<-tapply(yvec,trtvec,mean)
varvec<-tapply(yvec,trtvec,var)
nvec<-table(trtvec)
error.df<-nvec[1]+nvec[2]-2
pooled.var<-((nvec[1]-1)*varvec[1]+(nvec[2]-1)*varvec[2])/error.df
diff12estimate<-ybar[1]-ybar[2]
stderr<-sqrt(pooled.var*((1/nvec[1])+(1/nvec[2])))
tratio<-diff12estimate/stderr
twosidedP<-2*(1-pt(abs(tratio),error.df))
tcrit<-qt(1-alpha/2,error.df)
lower<-diff12estimate-tcrit*stderr
upper<-diff12estimate+tcrit*stderr
out<-data.frame(diff12estimate,stderr,tratio,twosidedP,lower,upper,alpha)
out
}