Distribution of the standard deviations of the spectral and luminosity types of the multiply observed OB stars. The x-axis represents the standard deviation of the spectral types classified by MKCLASS in different observations, and the y-axis represents the standard deviation for these luminosity types. The numbers in the x-axis represent the spectral subtype, such as 0.1-0.9 for O1-O9 and 1.0-1.9 for B0-B9. The numbers in y-axis stand for luminosity subtypes, i.e., 1-5 for I-V.