Correlations between s₂ and E (left panel), s₂ and ⁵⁶Ni mixing (middle panel), and Fe IIλ5169 line velocities at 50 days post-explosion and E (right panel). The mixing of ⁵⁶Ni within the ejecta is given as a fraction of the pre-SN mass. The yellow and red markers refer to the results obtained from the CSP-I SN II sample (yellow markers indicate gold events). Each subplot contains the number of events (N), the Pearson correlation coefficient (ρ), and the probability of detecting a correlation by chance (P) using the full sample. Results from the models are colour-coded based on different physical parameters. The physical parameters not being varied –if they do not appear in the plot– are fixed at M_ZAMS = 15 M_⊙, M_Ni = 0.04 M_⊙, and ⁵⁶Ni mixing = 0.5. Some observations fall outside the range of the model parameters because of the fixed physical parameters. Changes in the fixed values represent different ranges of model parameters.