The common practice to include firm fixed effefcts in corporate finance research may eliminate the explanatory power of important economic factors that are persistent. To illustrate this point, we review the intuitive R&D-patent relation in recent studies and surprisingly find that R&D input does not always positively explain patent output. This missing link can be attributed to the persistence of R&D and patents that causes the between-firm variation to be absorbed by firm fixed effects. We use Hausman-Taylor and advanced machine learning methods to recover or to modify the firm fixed effect models. We find that Hausman-Taylor correction and Machine learning-based models restore the positive R&D-patent relation. These methods thus offer a second opinion for empirical researchers working with explanatory variables that strongly correlate with between-individual unobservables.