This paper examines response error differences between self- and proxy respondents in a major and relatively new Census Bureau household survey--the Survey of Income and Program Participation (SIPP). Standard SIPP procedures call for self-response interviews with all adult members of sampled households if possible, but allow proxy interviews for those not present (or unable or unwilling to be interviewed) at the time of the interviewer's visit. As a result of these respondent rules, a substantial proportion of SIPP interviews are conducted by proxy. The large number of proxy interviews, coupled with concerns about the quality of SIPP data, have led to questions about the contribution of proxies to SIPP data quality problems. If proxies were found to contribute disproportionately to SIPP's quality problems, the survey's designers and sponsors would have to consider imposing more stringent self-response rules.