In order to better understand how social support affects successful aging, Dr. Kind works with a group of senior citizens in a local assisted living facility. They are all 70 years old when she first meets with them, but Dr. Kind is not interested in studying the differences in age at this first datapoint. She instead wants to see how the individuals change over time in relation to how many friends and family members they have actively involved in their lives. Dr. Kind assesses the individuals every two years for 10 years or until death occurs. Which research design is this?