Shockingly little is known, with certainty, about urban migration, though some estimates are that it may account for between 40 and 60 per cent of annual urban population growth.

• 虽然有些估计数据表明,流入到城市的人口占城市人口年增长率的40%25到60%25之间,但是,关于人口流动的确切情况,人们所知甚少。