Some efforts of the Bureau of the Census to measure and evaluate response errors are described. Particular attention is devoted to the October, 1948, pretest of Census procedures and the measurement of response errors of the various procedures. The procedures were: (1) Self-enumeration, using a household schedule. The schedule was a single sheet of paper 18″ × 24Prime; with 10 columns of questions for the various members of the household—two columns of questions for each individual repeated five times—on one side, and a letter to the household and questions on housing on the other side. (2) Self-enumeration using a 4 page booklet, the page size being 9Prime; × 11Prime;, with Individual Enumeration Forms (9Prime; × 11Prime;) inserted for each member of the household. The questions were printed front and back in two columns on the Individual Enumeration Form. (3) Obtaining information from respondents by direct interview. (4) Obtaining basic information by direct interview and leaving schedule with additional questions for respondent to fill out, the schedule to be picked up on a second visit to the household.