A child experiences a serious and rare illness at three months old. A researcher studies this child until adulthood to learn about the impact of the illness on his family. What method is the researcher using?

The researcher is utilizing a case study method in this scenario. This approach involves an in-depth analysis of a single individual or a small group of individuals, providing detailed insights into their experiences, behaviors, and the impact of specific conditions or events—in this case, a serious illness experienced by a child.

By focusing on this particular child and examining how the illness has affected him and his family over time, the researcher can gather rich qualitative data that would not be possible in studies involving larger populations. A case study allows for a comprehensive exploration of the unique circumstances surrounding the child and offers insights that might inform understanding of similar cases, even though it may not yield generalizable findings to a larger population.

In contrast, longitudinal studies involve tracking the same individuals over an extended period to observe changes and developments, while cross-sectional studies examine different individuals at a single point in time to identify patterns or differences across groups. The experimental method focuses on manipulating variables to determine cause-and-effect relationships, which is not the focus here. Therefore, the case study method is the most suitable choice given the detailed investigation of one individual over time.