GDP measures the market value of the goods and services a nation produces. Unpaid work that people do for themselves and their families isn't traded in the marketplace, so there are no transactions to track. Surveys asking people how they spend their time can be used to estimate household production. But the United States only began collecting these data annually in 2003, and many countries have never done a nationally representative survey. The lack of reliable data influenced the decision to leave household production out of GDP in the internationally accepted guidelines for national accounting.