Comparison between the theoretical and measured ratios of O iv: a) R₁; b) R₂; c) R₃; d) R₄. The theoretical ratios have been obtained from the three datasets: CHIANTI, denoted as ch; revised, denoted as rv; and Aggarwal & Keenan (2008), denoted as ak. The filled circles indicate the observations of April 2007 taken from Muglach et al. (2010), while the crosses, with the error bars, represent the observations of April 2009. The open circles show where the observational points of April 2007 must be plotted if the CHIANTI data are adopted as in Muglach et al. (2010). The intensity units are photons cm^-3 s^-1. The adopted electron density is N_e = 10¹⁰ cm^-3. The vertical lines are at the O iv temperature formation, log T_m = 5.11. Note that the ch, rv and ak data are basically identical for the ratios R₃ and R₄ shown in panels c) and d).