Health insurance marketplace  In the United States, health insurance marketplaces, also called health exchanges, are organizations in each state through which people can purchase health insurance. People can purchase health insurance that complies with the Patient Protection and Affordable Care Act (ACA, known colloquially as "Obamacare") at ACA health exchanges, where they can choose from a range of government-regulated and standardized health care plans offered by the insurers participating in the exchange. ACA health exchanges were fully certified and operational by January 1, 2014, under federal law. Enrollment in the marketplaces started on October 1, 2013, and continued for six months. 8.02 million people had signed up through the health insurance marketplaces. An additional 4.8 million joined Medicaid. Enrollment for 2015 began on November 15, 2014 and ended on December 15, 2014. As of April 14, 2020, 11.41 million people had signed up through the health insurance marketplaces. Private non-ACA health care exchanges also exist in many states, responsible for enrolling 3 million people. These exchanges predate the Affordable Care Act and facilitate insurance plans for employees of small and medium size businesses. Health insurance exchanges in the United States expand insurance coverage while allowing insurers to compete in cost-efficient ways and help them to comply with consumer protection laws. Exchanges are not themselves insurers, so they do not bear risk themselves, but they do determine which insurance companies participate in the exchange. An ideal exchange promotes insurance transparency and accountability, facilitates increased enrollment and delivery of subsidies, and helps spread risk to ensure that the costs associated with expensive medical treatments are shared more broadly across large groups of people, rather than spread across just a few beneficiaries. Health insurance exchanges use electronic data interchange (EDI) to transmit required information between the exchanges and carriers (trading partners), in particular the "834" transaction for enrollment information and the "820" transaction for premium payment. Health exchanges first emerged in the private sector in the early 1980s, and they used computer networking to integrate claims management, eligibility verification, and inter-carrier payments. These became popular in some regions as a way for small and medium-sized businesses to pool their purchasing power into larger groups, reducing cost. An additional advantage was the ability of small businesses to offer a range of plans to employees, allowing them to compete with larger corporations. The largest such exchange prior to the ACA is CaliforniaChoice, established in 1996. By 2000, CaliforniaChoice's membership included 140,000 individuals from 9000 business groups. Obamacare maintained the concept of health insurance exchanges as a key component of health care.