For real persons at risk for Alzheimer's disease, a neurodegenerative disease, or brain aging, a measurement's rate of change can be characterized during or following the real persons' treatment with disease-preventing or neurological age-slowing therapy. For hypothetical persons similar to the real persons at risk for these conditions but who are not so treated, the measurement's rate of change can be characterized over a like time interval. The disease-preventing or age-slowing therapy's efficacy is suggested by a smaller measurement rate of change over the like time interval in the real persons treated than in the hypothetical persons not so treated, even in the absence of clinical decline over the time interval. Measurements of neurodegenerative disease progression will have significantly higher rates of change in persons clinically affected by or at risk for the disease than in those persons at lower risk for the neurodegenerative disease.