Cumulative density functions (CDFs) of four StarTrack models versus effective spins χ_eff. Solid color lines correspond to χ_eff CDF of detectable events for each model. The black line shows the χ_eff CDF of median likelihoods of the O1/O2 LVC events, and the gray region bounds CDFs of the 5th and 95th percentiles of the likelihoods from LVC parameter estimation. In the corresponding colored shaded regions, we show the 90% range of 5000 mock observed CDFs under each model. To generate a mock observed CDF, we draw 10 χ_eff values from the detected population under a given model and add random Gaussian noise with σ = 0.05, which is approximately the uncertainty in χ_eff likelihoods from the events of GWTC-1 (we resample any samples with |χ_eff| > 1 after adding noise).