Courtesy of Dr David Dickie, this image shows that the appearance of brain structure in a group of subjects diagnosed with Alzheimer’s disease (AD) versus a normal control group changes based on the statistical method used.
The image on the left shows voxel-wise parametric effect size and the image on the right shows voxel-wise NONparametric effect size (red areas = higher effect sizes > 0.75). If the voxel-wise data were normally distributed then the parametric effect size would equal the NONparametric effect size. However, these voxel distributions are not normally distributed therefore the parametric method appears to be artificially increasing effect size between the AD and normal control groups. This suggests that differences in brain structure between AD and normal ageing may be more subtle than suggested by parametric methods (a finding often reported in clinical practice).