#Two-Sample Test of Proportions
  prop.test(x=c(56,16), n=c(900,411), correct=F) 

#OR

#Chisquare Test
  x=c(56,16)   # No. of "successes"
  n=c(900,411) # No. of trials
  n-x          # No. of "failures"
  
  tab = matrix(c(x,n-x), nrow=2, byrow=TRUE)
  tab
    rownames(tab) = c("Yes"    , "No")      #Name rows and
    colnames(tab) = c("Prevast", "Placebo") #cols (optional)
  tab
  
  chisq.test(tab, correct=FALSE)            #Alternative test of proportions

  chisq.test(tab, correct=FALSE)$observed
  chisq.test(tab, correct=FALSE)$expected
























o1 = 278; o2 = 122
e1 = 300; e2 = 100
x2.s = (o1-e1)^2/e1 + (o2-e2)^2/e2
pchisq(x2.s, 1)
1-pchisq(x2.s, 1)