First, the national definition of migration often differs from the internationally recommended definition.

Beyond this, these estimates cannot be included in administrative data collection that measures legal immigration.

At the same time, the increase in the number asylum claims makes the comparison and estimation of flows difficult.

Relative to other demographic phenomena, like births and deaths, migration is assuming increasing importance in measurements of population change.

Unlike other demographic phenomena, however, migration flows pose particular challenges to the production and measurement of reliable and widely comparable statistics.

This example makes clear that among the challenges of improving the comparability of international migration statistics, harmonizing the concepts and definitions of migration are no less important than resolving the problems of poor reliability of data collection systems.

How is it possible to explain such large differences between statistical figures supposed to measure the same migration flow?This would do much to improve the reliability and comparability of statistics.There remain, nevertheless, a number of unique challenges that must be overcome if migration statistics are to be truly reliable.The European Model The Eurostat measurement of flows between Denmark and Germany in 1998 illustrate the difficulty of measuring migration flows.When the registered level of immigration in Germany is compared with the Danish equivalent, it appears that Denmark underestimates immigration flows by 50 percent.In fact, the definitions used in Germany and in Denmark are very different: Germany considers every entry an "immigration" after seven days, while Denmark requires an intended duration of three months.