Recent work in predictive modeling has called for increased scrutiny of how models generalize between different populations within the training data. Using interaction data from 69,174 students who used an online mathematics platform over an entire school year, we trained a sensor-free affect detection model and studied its generalizability to clusters of students based on typical platform use and demographic features. We show that models trained on one group perform similarly well when tested on the other groups, although there was a small advantage obtained by training individual subpopulation models compared to a general (all-population) model. Lastly, we perform a series of simulations to show how generalizability is affected by sample size. These results agree with our initial analysis that individual subpopulation models yield a small advantage over all-population models. Additionally, we show that training sizes smaller than 1,500 yield unstable models which make generalizability difficult to interpret. We discuss applications of this work in the context of developing large-scale affect detection models for diverse populations. [For the full proceedings, see ED599096. For the corresponding grantee submission, see ED595455.]