Which Portfolios? The Construction Dependence of Factor Model Performance
How the way you test stock models changes which one wins
A finance researcher tested five different models for predicting stock returns using randomly constructed portfolios, and found that which model performs best depends heavily on how the test is set up—including how stocks are weighted and how often trades happen. The model ranked best in one test design (buy-and-hold) ranked third in another (daily rebalancing), suggesting researchers' conclusions about which model to use could flip based on choices made during testing.
Investment firms and researchers use these factor models to decide which stocks to buy and how to build portfolios worth billions of dollars. If a model's apparent superiority disappears when you change the testing method, it means investors could be making costly decisions based on results that don't generalize to real trading. This work shows that researchers need to test models across multiple construction methods before claiming one is truly better than another.