CFH and Dystonia: Assuming MAF of 0.4, odds ratio 1.3, additive model, and prevalence of 430 per million in dystonia, it would need more than 1800 cases to achieve an 80% power.21 If the odds ratio was as high as with the Complement Factor H in macular degeneration,22 the cohort of 212 cases with same assumptions would have a power of almost 100% to detect the SNP.