Globally, international migration flow data are poor. Putting aside issues of data quality and definitional nuances for one moment and concentrating just on coverage, it is fair to say that little is known of the annual flows of people between the circa 250 countries which make up the political map of the world. If we imagine a large two-dimensional data matrix with each of the world's counties heading the rows (as origins) and columns (as destinations), with the interior cells of the matrix containing counts of migrants moving between each country over the course of a year, then for any random year, the chances that there are non-zero data filling even a small proportion of the cells will be low.
Even if the matrix happens to be representing flows in the first or second year of a decade when, curiously, many of the countries that still carry out a full population census do so almost in synchronisation with each other, coverage would be patchy. The matrix may be slightly more populated with data than in other years, but it would very likely be characterised by vertical lines of data with far fewer horizontal lines – this is because Censuses are quite good at counting people into countries, but rather poor at counting people out (it is hard to count someone after they have left).
Some sections of the matrix, even in a non-census ...