Two-Sample Test for Proportions

The Null hypothesis of "No Effect" can be re-written as:
H0: "Proportion of Colds among Placebo users = Prop. of Colds among Vitamin C users"
If H0 is true we could estimate the common proportion P of colds by

EP = (total no. of colds) / (total no. of Skiers) = 0.17

The z test statistic is then given by:

z = (obs. difference)/ SE


(obs. difference) = 31/140 - 17/139 = 0.1

and the SE for the difference is given by the sqrt-law:

SQRT( EP*(1-EP)*(1/140 + 1/139) ) = 4.57e-4

Therefore the observed test statistic is:

z = 0.1 / 4.57e-4 = 2.19

From the normal table we obtain the area between -2.19 and 2.19 to be about 97.2%. This means that the tail area from 2.19 to the right is (100%-97.2%)/2 or about 1.4%

We conclude that there is evidence in the observed data against the NULL hypothesis that "Vitamin C" has no effect on Colds. The p-value is 1.4%.

Solution Using the Chi-square Test

Expected table under independence (the null)

                  Cold    No Cold   Totals     
Placebo 	  24.1	  115.9	    140        
Vitamin C	  23.9	  115.1	    139        
Totals		  48	  231	    279 

Therefore: Chi-square = 4.8 , and the p-value from the Chi-square table with 1 degree of freedom is less than 5% so we reject the null. We conclude that vitamin C shows a significant effect.