Before revision:  A global dispersion force accrues from competition between different locations and leads to the formation of multiple economic clusters , or cities . A local dispersion force is caused by crowding effects within each location and induces the flattening of each city.
After revision:  A global dispersion force accrues from competition between different locations and leads to the formation of multiple economic clusters (or cities) . A local dispersion force is caused by crowding effects within each location and induces the flattening of each city.