Dependence of central SFR indicators on spiral arm strength (2 + log s_arm). Panels a–c: plot the D_n(4000), Hδ_A, and C(sSFR) against arm strength, respectively. The bin color encodes the number of galaxies in each bin, scaled by the color bar next to the panel. The Pearson correlation coefficient and the corresponding p value are denoted at the top. The confidence ellipses are obtained using the PCA technique described in the text, and they contain approximately 95% of the data. As these ellipses could be distorted due to the different dynamical ranges of the x and y axes, a best-fitted straight line indicating the direction of main principal component is plotted.