The prerequisites for the referendum included a census, which was used to define how wealth and political power will be apportioned between regions. The census was the basis of a voter registration process, which was also used for the national elections in 2010, which in turn set the stage for the referendum. The census was delayed three times. Problems included disagreements between the north and south over what they are obliged to do by the Naivasha Agreement, funding difficulties and an enormous logistical challenge. In the south, unmapped minefields from the war continue to make movement difficult, while up to 5,000,000 Sudanese are nomadic. Up to 2,000,000 internally displaced persons from the south remain in camps around Khartoum, in the centre of the country, whilst refugees remain in Uganda and Kenya. A further complication results from the conflict in Darfur to the west, where civilians who have fled attacks refuse to take part in census out of fear that the government would use the results against them. Darfuri rebel groups are unanimous in their denunciation of the planned census, while the Justice and Equality Movement group has threatened to attack any census-taker.