Looping questions are used to collect data about several similar events, such as employment spells, retirement accounts, or marriages. The loops gather information about the number of events experienced as well as details about each one. The questions require respondents to think hard to recall each event and are often lengthy and repetitive. Looping questions can be asked in two formats, and which format a survey uses may affect the quality of the data collected.We develop hypotheses about the effects of format on measurement error in looping questions and test the hypotheses using experimental data from a web survey with a link to administrative records. Results show that one format collects more accurate event reports, but the other format provides higher quality data to the follow-up questions. We conclude with guidance for those who write survey questions and those who rely on survey data for substantive analyses.