In 1983 the Bureau of the Census initiated the Survey of Income and Program Participation (SIPP), a survey designed to measure the economic and social changes in persons lives over time. The survey relies on a national household sample, with in-person interviews of all persons ages 15 and over. Households are in sample for a total of 32 months and are visited by an interviewer every four months (for a total of 8 interviews). During each interview (referred to as a "wave"), subjects are asked questions about their employment, economic situation and program participation for each of the preceding four months.