Oxygen abundance vs. gas fraction relation for nearby dIrrs and BCDs. Filled blue and open magenta circles are data from Vaduvescu et al. (2007), and they represent BCDs and dIrrs respectively; open magenta triangles are dIrrs observed by Lee et al. (2006); filled cyan triangles and squares are the Irrs observed by Pilyugin et al. (2004) and Garnett (2002), respectively. The solid line shows the prediction of the closed box model (Z = y_Zlnμ^-1) with an effective yield y_Z = 0.007 corresponding to the Salpeter IMF and Woosley & Weaver (1995) nucleosynthesis.