The definition of a small farm has varied over time and by country. Agricultural economists have analyzed the distinctions among farm sizes since the field's inception. Traditional agricultural economic theory considered small farms inefficient, a stance that began to be challenged in the 1950s. An overview of research published by the World Bank in 1998 indicated that the productivity of small farms often exceeded that of larger ones. The definition of a small farm has varied over time and by country. Agricultural economists have analyzed the distinctions among farm sizes since the field's inception. Traditional agricultural economic theory considered small farms inefficient, a stance that began to be challenged in the 1950s. An overview of research published by the World Bank in 1998 indicated that the productivity of small farms often exceeded that of larger ones. Several definitions of the term have been formulated in legislation. In 1977 the US Congress, via the Food and Agriculture Act of 1977, defined a small farm as one with sales under $20,000. At the time these comprised 70% of farms in the US. The Act sponsored additional research on small farming operations by US land grant universities and their extension services and mandated that an annual report on these activities be issued by the US Secretary of Agriculture. A 1997 study by the United States Small Farms Commission defined small farms as those with less than $250,000 in gross receipts annually on which day-to-day labor and management are provided by the farmer and/or the farm family that owns the production, or owns or leases the productive assets. In 2000, such farms accounted for about 90% of the more than 2.1 million U.S. farms, but only about 40% of U.S. farm production.