Which study design is relatively rare due to its expensive nature?

The study design that is considered relatively rare due to its expensive nature is the sequential design. This design combines elements of both cross-sectional and longitudinal studies, allowing researchers to analyze data from different cohorts across multiple points in time. The inclusion of multiple groups over an extended timeframe necessitates greater resources, such as funding for participant recruitment, data collection, and data management, which can substantially increase the overall cost.

Longitudinal studies, while also costly, typically focus on a single group over time, making them somewhat more feasible for researchers. Cross-sectional studies involve only one group at a single point in time and are less resource-intensive. Microgenetic studies, which focus on short-term changes and observe participants during a specific developmental transition, require detailed observations over a brief period but do not usually involve the extensive duration and breadth that sequential designs demand. This combination of factors makes sequential designs less common in practice.