chisquare.R

#http://www.r-tutor.com/elementary-statistics/goodness-fit/chi-squared-test-independence

library(MASS)
tbl = table(survey$Smoke, survey$Exer)
tbl

##        
##         Freq None Some
##   Heavy    7    1    3
##   Never   87   18   84
##   Occas   12    3    4
##   Regul    9    1    7

table(survey$Smoke)

## 
## Heavy Never Occas Regul 
##    11   189    19    17

dim(survey)

## [1] 237  12

#Test the hypothesis whether the students 
#smoking habit is independent of
#their exercise level at .05 significance level.
chisq.test(tbl)

## Warning in chisq.test(tbl): Chi-squared approximation may be incorrect

## 
##  Pearson's Chi-squared test
## 
## data:  tbl
## X-squared = 5.4885, df = 6, p-value = 0.4828

#As the p-value 0.4828 is greater than the .05 significance level, we do not reject the null hypothesis that the smoking habit is 
#independent of the exercise level of the students.
ctbl = cbind(tbl[,"Freq"], tbl[,"None"] + tbl[,"Some"]) 
ctbl 

##       [,1] [,2]
## Heavy    7    4
## Never   87  102
## Occas   12    7
## Regul    9    8

chisq.test(ctbl)

## 
##  Pearson's Chi-squared test
## 
## data:  ctbl
## X-squared = 3.2328, df = 3, p-value = 0.3571

#As the p-value 0.3571 is greater than the .05 significance level, we do not reject the null hypothesis that the smoking habit is 
#independent of the exercise level of the students.


#The warning message found in the solution
#above is due to the small cell values in
#the contingency table.