Where does this 30% number come from?
It is a standard that originated with a well-cited article from The New England Journal of Medicine published in 1988 by Laupacis et al, "An assessment of clinically useful measures of the consequences of treatment". 30% or more change in patient-reported scores (using a validated measurement tool) is considered clinically meaningful change.
That is often interpreted as any clinical outcome that is less than 30% is not considered meaningful (as a metric--I am sure 29% improvement meant much to a patient). And 30% or more improvement in function is considered meaningful. So far I see this quoted in RCTs (randomized controlled trials), clinical effectiveness trials, and literature reviews. Literature reviews that can show a minimum of 30% change (usually an improvement in symptoms or function) is useful when providing evidence of clinical effectiveness when making a change in policy whether it is facility-level, state-level, or national level policy.
Research journals, literacy, levels to consider when publishing or perusing