Migration is also more difficult to measure than fertility and mortality. Most countries do not have an easy and accurate way to track population movements. Every year, millions of Americans move to a new home, but not all such residential mobility is classified as migration. Neither are temporary moves for work or leisure. Migration refers only to the movement of people across a territorial boundary for the purpose of changing their place of usual residence.
International migration involves movement across a national border.
Internal migration describes moves within a country. In the United States, people must move across a county line to be an official migrant. Net migration, the difference between the number of people moving in and out, may be positive or negative. Between 1990 and 1997, Florida had a net gain of 1.3 million people through migration, while New York state suffered a net loss of 673,000 people. The United States as a whole experienced a net immigration of 5.6 million people from abroad between 1990 and 1996.