Because migration is generally equated with the entry of foreigners, few countries gather information on these four types of migrant flows, although they occur in all countries to varying degrees.
因为人们普遍把移徙同外国人入境等同起来，很少有国家收集这四类移徙者流动情况的资料，虽然所有国家在不同程度都有过这些移徙。