The Reverend Henry Whitehead was skeptical of John Snow's conclusion about the Broad Street pump. After the Broad Street cholera epidemic ended, Whitehead set about trying to prove Snow wrong. (The history of the event is detailed here.) He realized that Snow had focused his analysis almost entirely on those who had died. Whitehead, therefore, investigated the drinking habits of people in the Broad Street area who had not died in the outbreak. What is the main reason it was important to study this group? 1) If Whitehead had found that many people had drunk water from the Broad Street pump and not caught cholera, that would have been evidence against Snow's hypothesis.
2) Survivors could provide additional information about what else could have caused the cholera, potentially unearthing another cause.
3) Through considering the survivors, Whitehead could have identified a cure for cholera.