How to forecast the size of internal migration in the future Note that their rates during the previous censuses oscillate and do not go in one direction
Except if you can explain those variations by events that are unlikely to happen in the future (the Balkan war for instance), it is reasonable to assume that those past variations in migration flows will continue in the future. Thus, using the average of past years appears as a reasonible choice. If you can model migration flows by other factors (wage differences, unemployment, distance, etc.) you can maybe set more sophisticated assumptions, but you'll need both good data on the past (that you don't have) and you'll need to set assumptions for the future of those factors, which brings a much bigger source of uncertainty.