Available evidence suggests that this tendency is likely to have contributed to the rising share of workers on fixed-term contracts in OECD countries (see Box 2.3). Are the partial reforms of EPL in the 1990s - whereby hiring on temporary contracts was largely deregulated while maintaining stringent restrictions on regular contracts - responsible for this expansion of non-permanent - and often precarious - forms of employment? Identifying a significant relationship between changes in regulations and the stock of workers on fixed-term contracts is typically difficult (see e.g. OECD, 2004, 2010), in particular because substitution across types of contracts is likely to occur only over time through the process of hiring and separations (see e.g. Boeri, 2011).