However, recent work has questioned the traditional view and argued that duality can also be present in the informal sector itself (Pages and Stampini, 2007, Fields, 2008). Given that worker mobility is usually correlated with wage differences between sectors, the size of the wage gap itself is also an indicator of labour market segmentation. Gong and van Soest (2002) show that in urban Mexico wage differentials increase with educational level, especially in the formal sector.