Moreover, internal and international migrations can be linked in a stepwise migration process, whereby for example an internal move towards larger cities then leads to international migration. According to Lee, in addition to push and pull factors, migration decisions are affected by a set of intervening obstacles or constraints that may prevent people from migrating or at least make migration more difficult and/or costly. These decisions are subject to constraints or obstacles that may prevent people from moving, such as the cost of migrating and the distance to be travelled, among others.