Distributions of abundance residuals (Δlog(O/H); best-fit linear gradient subtracted). The histograms correspond to the left-hand-side vertical axes (probability distribution function (PDF)), and the curves correspond to the right-hand-side vertical axes (cumulative distribution function (CDF)). The top row compares the inter-arm H II regions with those in the four different spiral arm structures – S1–S4. The bottom row compares the inter-arm H II regions with S1 + S2 + S3 + S4 (all spiral arm structures; first panel from the left) and S1 + S2 + S3 (without S4; second panel from the left). The p-values from two-sided Kolmogorov–Smirnov and Anderson–Darling tests are labeled in each panel (p_KS and p_AD). Details are discussed in Sect. 4.2.