Because it does not conform to national accounting and overlaps with the modern sector, studies (such as those of PARSTAT) still underestimate its real size. In practice, this economy is defined essentially by default: "informal" covers any economic activity that does not comply with the obligations to register, keep accounts and pay taxes - even in recent studies, such as PARSTAT. The rapid urbanisation of sub-Saharan Africa is not based on the demand for industrial labour or, more generally, the supply of modern employment arising from industrialisation and external trade.