“…2 A possible explanation for this is that most studies (especially in Europe) rely on relatively small sample sizes of individuals who work PT making difficult the heterogeneity analysis. We have identified the following exceptions: Mocan and Tekin, 2003, analyze the nonprofit sector dimension; O'Connell and Gash, 2003, focus on differences between skilled and unskilled workers; and Ferber and Waldfogel, 1998;Rodgers, 2004;Booth and Wood, 2008;Hirsch, 2005;O'Dorchai et al, 2007;and Mumford and Smith, 2007, study the gender dimension (or focus on male workers).…”