Spectral indices α_mm vs. the cavity radius for the observed transition disks. Data from our PdBI observations and available data from the literature are plotted (data in Tables 1 and 2). Triangle points correspond to sources for which the cavity radius is inferred from SED modelling, while circle points correspond to resolved cavities by continuum observations. The uncertainties on the cavity radii are taken to be 40% of the mean value in the case of SED modelling and 5 AU for the resolved cavities. The dashed line corresponds to the linear regression from the data using the method introduced by Kelly (2007). The shaded regions show the 95% (2σ) confidence interval (lighter) and the 68% (1σ) confidence region (darker) on the regression line. The dot-dashed line represents the best linear regression from the models described in Sect. 4.