For a research, we are using county level data for fatal car accidents. However, this data is very sparse for counties with small populations. I would like to know if there is a guideline for merging (aggregating) data of counties with small populations. In other words, is there any unit of analysis that is finer than states but larger than counties with considerable population.
Census has Metropolitan and Micropolitan Statistical Areas
as well as Combined Statistical Areas (CSA):
and CBSA (Core Based Statistical Areas):