Moreover, some analysts have stressed that informal firms generate efficiency losses because, by avoiding taxes, they rob more productive formal competitors of market shares (Farrell, 2004). Data on micro-firms in Mexico from 1992 to 2008 show that the percentage of female owners increased more in the informal sector than in the formal one (Figure 29.2). Source: OECD Secretariat estimates based on six waves of INEGI Encuesta Nacional de Micronegodos (ENAMIN). The data are representative of Mexican urban areas.