Maximum Hα-to-FUV flux ratios of stellar clusters predicted by our models. The black curves show F_Hα/f_λFUV against IMF slope (α) and truncation mass (M_u) for the set of steep models and the set of truncated models, respectively. The first dot from the left is from the model with the standard Salpeter IMF (α = 2.35, M_u = 100 M_⊙). The horizontal red line and shaded area show the maximum observed F_Hα/f_λFUV in our catalog and its errors. The dashed black horizontal line is the limit of Hα brightness that we adopted. The change in the observed flux ratio if is assumed to be larger by 0.1 mag is shown with a black arrow. Only models with α < 2.8 or M_u > 60 M_⊙ can generate clusters with as high a F_Hα/f_λFUV as are observed.