Question: When did the man dubbed  "the most influential private citizen in the America of his day" hire Elmo Roper?
Sentence (from "Elmo Roper"): Elmo Burns Roper, Jr. (July 31, 1900 in Hebron, Nebraska – April 30, 1971 in Redding, Connecticut) was a pollster known for his pioneering work in market research and opinion polling.
Relevant evidence: No