Data from the Salk vaccine field trial suggest that in 1954, the school districts in the NFIP trial and in the randomized controlled experiment had similar exposures to the polio virus.

(a) The data also show that children in the two vaccine groups (for the ran- domized controlled experiment and the NFIP design) came from families with similar incomes and educational backgrounds. Which two numbers in table 1 (p. 6) confirm this finding?

(b) The data show that children in the two no-consent groups had similar fam- ily backgrounds. Which pair of numbers in the table confirm this finding"?

(c) The data show that children in the two control groups had different family backgrounds. Which pair of numbers in the table confirm this finding?

(d) In the NFIP study, neither the control group nor the no-consent group got the vaccine. Yet the no-consent group had a lower rate of polio. Why?

(e) To show that the vaccine works, someone wants to compare the 44/100,000 in the NFIP study with the 25/100,000 in the vaccine group. What's wrong with this idea? plo47