Traditionally, as reflected in methodology textbook reviews of the survey nonresponse literature, interviews have been thought to achieve substantially higher response rates than mailed questionnaires. The paper describes a correlational design for assessing the typical response difference between these two forms of data collection, after controls for other factors known to affect survey response. Attention is given to change over time in the response difference between interviews and questionnaires. In the primary data set, the net response due to data collection amounts to some 7.5 percentage points, in the middle (30–70) range of survey response. The analysis quantifies the decline over time in interview response, and shows mailed questionnaires to be free from decline. A second data set reproduces the essential findings.