#Biostatistics 160

#ZAR EXAMPLE 8.1
ZAR=read.table("ZarEX8.1.txt", na.strings="NA")
ZAR
attach(ZAR)
X1=na.omit(gpB)
X1
X2=gpG
X2
n1=length(X1)
n1
n2=length(X2)
n2
X1bar=mean(X1)
X1bar
X2bar=mean(X2)
X2bar
s1=sqrt(var(X1,na.rm=TRUE))
s1
s2=sqrt(var(X2))
s2
sp=((n1-1)*s1^2+(n2-1)*s2^2)/(n1+n2-2)
sp

#PERFORMING TWO-SAMPLE TWO SIDED t-TEST:
t.test(X1,X2,alternative="two.sided",var.equal=TRUE, conf.level=0.95)
detach(ZAR)

#CHECK THE FOLLOWING DATASET FOR A MORE
#EFFICIENT WAY TO INPUT DATA USING read.table()

ZAR2=read.table("ZarEX8.1R.txt")
ZAR2
attach(ZAR2)
X1=data[group=="gpB"]
X1
X2=data[group=="gpG"]
X2

#PERFORMING TWO-SAMPLE ONE SIDED LOWER TAIL t-TEST:
t.test(X1,X2,alternative="less",var.equal=TRUE, conf.level=0.95)

#PERFORMING TWO-SAMPLE ONE SIDED UPPER TAIL t-TEST:
t.test(X1,X2,alternative="greater",var.equal=TRUE, conf.level=0.95)