“I know NIAID’s prior approval is needed to reduce an investigator’s level of effort on a grant by 25 percent or more. Would a decrease from 50 percent effort to 30 percent effort need prior approval?”

Yes, the requirement for prior approval is based on percent change in level of effort, not the drop in percentage points of level of effort. In this case, a decrease from 50 to 30 percent effort amounts to a 40 percent reduction [(0.5-0.3) / 0.5 = 0.4].

This is conceptualized more easily if you consider the effort level in person months. A change from 6.0 person months (50 percent effort on a 12-month calendar year) to 3.6 person months (30 percent effort on a 12-month calendar year) is a decrease of 40 percent [(6.0-3.6) / 6.0 = 0.4].