Strengths and Limitations
MCI
Strengths of our trial included the selection of a fractional factorial design to test the sparsity-of-effects principle and, therefore, expose interaction effects over low-order interactions, such as the effect of vitamin D. Additional strengths were the use of a primary outcome sensitive to cognitive changes in MCI,Our trial had several limitations. The early termination imposed by COVID-19 pandemic restrictions impeded us from reaching our target sample size and provoked dropouts in those already enrolled, which likely affected our power to find a statically significant effect, for instance, for the exercise intervention alone. Most participants were vitamin D sufficient at baseline, which may have reduced any potential effect of the vitamin D intervention. Additionally, our sample was largely White and with postsecondary education, which reduces the generalizability of our results. Finally, we did not adjust our analyses for having 2 primary end points, which may have increased our type I error.