Top: comparison of the N/O ratio between the H i and H ii region for the sample of BCDs studied with FUSE, and for I Zw 18 with COS. The labels are defined as 1a) I Zw 18 (L04), 1b) I Zw 18 (A03), 1c) I Zw 18 (this study, with S as a tracer of O; Sect. 6.2), 2) NGC 1705 (Heckman et al. 2001), 3) SBS 0335-052 (Thuan et al. 2005), 4) Mark 59 (Thuan et al. 2002), 5) NGC 625 (Cannon et al. 2005), 6) I Zw 36 (Lebouteiller et al. 2004), 7) Pox 36 (Lebouteiller et al. 2009), 8) NGC 604 (Lebouteiller et al. 2006), 9) SBS 1543+593 (Bowen et al. 2005; Schulte Ladbeck et al. 2004). Bottom: ratio of the average metal abundance [M / H] = ⟨ [X / H] ⟩ between the H i and H ii regions. [M/H] is calculated two ways, averaging all elements observed in each phase (small diamonds), and averaging only the elements observed in both phases (large diamonds).