Power versus minimum detectable difference (MDD). Study power as a function of the MDD, computed as the percent difference for four instruments in 451 RA patients. The Item-Improved HAQ and PROMIS PF 20 show greater power across most MDD levels. The Item-Improved PF-10 is substantially more informative than the Legacy PF-10. The maximal power difference between the greater and lesser curves is with an MDD of 1 to 1.5 percent; instruments behave more similarly when the MDD is larger or smaller. Instruments that are more sensitive have their greatest advantages in detecting small differences; when differences are large, there is less difference between individual instruments. HAQ, health assessment questionnaire; PF, physical function; PROMIS, Patient-Reported Outcomes Measurement Information System; RA, rheumatoid arthritis.