The longitudinal research design which found that African-American respondents are less likely than Caucasian respondents to receive assistance or to use informal sources of health care was:

Asset and Health Dynamics Among the Oldest Old Study.

Explanation:The Asset and Health Dynamics Among the Oldest Old (AHEAD) is an auxiliary study that focuses  on data to solve  a wide  range of scientific questions in terms of late-life health changes, economic status defined by race, language fluency, religion, ethnicity etc

According to the American Economic Association,  the initial sample consisted of 7,447 respondents aged 70+, including 2,548 aged 80 and over, plus 775 younger spouses and there are constant follow-ups every two years. The Data found here has helped to find out that African-American respondents are less likely than Caucasian respondents to receive assistance or to use informal sources of health care.