The researchers looked at different models to see how well they explain stock returns in Germany. They found that a modified version of the Capital Asset Pricing Model (CAPM) with changing parameters does a better job than the original model. This new model, which includes the term spread as a factor, works almost as well as the Fama-French model. Tests showed that the new model is stable over time, unlike the Fama-French model. Overall, models that don't change with time are better at predicting stock returns in Germany.