Calibration of S_CaII to the Mt. Wilson scale using 43 stars with S_MW values from Duncan et al. (1991) and Baliunas et al. (1995a). The solid black line is the best linear fit with slope (a) and intercept (b) coefficients shown in the plot with respective errors obtained from the diagonal of the covariance matrix. The residuals standard deviation is 0.017. Errors are the standard deviations of the data. Errors for the indices in Baliunas et al. (1995a) were not published. In some cases the errors are smaller than the symbols.