Random forest regression analysis: Association between brain MRI measures and EF with a speed component. Free water in WMH and in NAWM were shown to have highest variable importance for EF with a speed component in a random forest regression. Abbreviations: FW = free water, WMH = white matter hyperintensities, NAWM = normal appearing white matter, tf = transformed, t = suffix for free water corrected tissue, AD = axial diffusivity, nBV = normalized brain volume, FA = fractional anisotropy, RD = radial diffusivity, WMHvol_norm_ln = normalized and log-transformed WMH volume, MD = mean diffusivity.