Consider the clinical trial of Tetanus Anti-toxin in Treatment of Tetanus. (hint: see reading material 4 for reference)

	Anti- Toxin (A)	No Anti- Toxin (B)
Alive	21	9	30
Dead	20	29	49
	41	38	79

1. Calculate the variance of the death rate differenceVar(pA-pB)between group A and B.

1. Use the time from 1st symptoms to admission as a stratifying variable,

A post-stratification was carried out and outcomes are shown in the following table. Calculate theVar(pA-pB)using the post-stratification. How much would one have gained in precision using post-stratification compared to non-stratification?

< 72 hours > 72 hours

	A	B
Alive	11	5
Dead	2	3
	13	8

	A	B
Alive	10	4
Dead	18	26
	28	30