* The author expresses with pleasure his indebtedness to Leo P. Crespi and to Fred W. Trembour for the privilege of introducing in September, 1952, the plan of duplicate samples into the public opinion surveys carried out by the American Embassy in Germany (then the U. S. High Commission in Bonn). In the original experimental work, and in earlier and later drafts of the manuscript and of the instructions for the procedure, I had the continual assistance of Josephine D. Cunningham, without whom this publication would have suffered indefinite delay.