The model-based estimation of primary nucleation rates from the probability distribution of induction time measurements has been used extensively in the literature; however, there has been no experimental validation of this approach via direct measurements of nucleation rates. In this work, we compared the nucleation rates obtained from the probability distribution model against that measured from in situ optical reflectance measurement (ORM) with p-aminobenzoic acid and L-glutamic acid as model compounds. Results reveal that the primary nucleation rates obtained from stochastic models are several orders of magnitude lower than those measured from a direct particle-counting technique. Although differences in fluid dynamics due to agitation and crystallizer geometry may have an effect, this tremendous discrepancy provides strong evidence that primary nucleation rates obtained from induction time measurements are not scalable and should not be used for interpretation of nucleation rates in industrial applications.