A surrogate marker is a biological indicator used in clinical research to substitute for a direct measure of how well a treatment works. It is often a laboratory measurement or a physical sign that can predict the outcome of a disease or the effectiveness of a therapy, such as blood pressure for heart disease. Surrogate markers are valuable because they can provide quicker results than waiting for actual clinical outcomes, like survival rates. For example, in cancer research, a decrease in tumor size can serve as a surrogate marker for the effectiveness of a new drug, helping researchers assess treatment efficacy more rapidly.