Distributions of abundance residuals, Δlog(O/H), for different inter-arm regions. The four spiral structures defined in Figs. 4 and 5 divide the galactic disk into three inter-arm regions. Each of the three panels compares two of the three inter-arm regions. “Inter-arm(S1, S2 + S3)” denotes the inter-arm region between the S1 and S2 + S3 spiral regions. Both histograms (left y-axis) and cumulative distribution functions (right y-axis) are shown. Under the null hypothesis that the two distributions have the same mean, the p-values derived from bootstrap are tabulated in Table 4.