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The United States of America (USA), commonly referred to as the United States (U.S.) or America, is a federal republic composed of 50 states, the federal district of Washington, D.C., five major territories, and various possessions. The 48 contiguous states and Washington, D.C., are in central North America between Canada and Mexico. The state of Alaska is in the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The territories are scattered about the Pacific Ocean and the Caribbean Sea. At 3.8 million square miles and with over 320 million people, the country is the world's third largest by total area and the third most populous. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The geography and climate of the United States are also extremely diverse, and the country is home to a wide variety of wildlife.
The land area of the contiguous United States is 2,959,064 square miles. Alaska, separated from the contiguous United States by Canada, is the largest state at 663,268 square miles. Hawaii, occupying an archipelago in the central Pacific, southwest of North America, is 10,931 square miles in area. The populated territories of Puerto Rico, American Samoa, Guam, Northern Mariana Islands, and US Virgin Islands together cover 9,185 square miles.
The United States has been a leader in scientific research and technological innovation since the late 19th century. Methods for producing Interchangeable parts were developed by the U.S. War Department by the Federal Armories during the first half of the 19th century. This technology, along with the establishment of a machine tool industry, enabled the U.S. to have large scale manufacturing of sewing machines, bicycles and other items in the late 19th century and became known as the American system of manufacturing. Factory electrification in the early 20th century and introduction of the assembly line and other labor saving techniques created the system called mass production. In the 21st century, 64% of research and development funding comes from the private sector. The United States leads the world in scientific research papers and impact factor. There have been numerous lists of oncology journals in United States and many other United Kingdom oncological journals which are running with good number of articles and good reputation.