Migration is the most complex and volatile demographic variable. Migration is selective. More educated and more adventuresome people are more likely than other people to move, for example. Migration is closely tied to the life cycle. People are most likely to move at certain stages of their lives, especially when they marry, divorce, or retire.
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.