The Actual Deferral Percentage (ADP) Test is a rule used for certain workplace retirement plans to make sure all employees—whether they earn a little or a lot—have a fair chance to save for retirement. It compares how much money employees contribute from their paychecks, looking at the average contribution rates of higher-paid employees versus lower-paid employees. If the plan favors higher earners too much, it may fail the test and have to return some contributions or make adjustments. Solo 401k plans usually don’t need to worry about this test, since they’re designed for self-employed individuals with no full-time employees other than a spouse.