(a) Strike a balance between the number of groups and the potential reduction in necessary sample size. In most cases, you will only want to deal with two or three groups.
(b) You may try different ways of grouping your service and choose one particular grouping based on its efficiency improvement and your administrative convenience.
(c) The grouping option may be used for administrative convenience alone:
(1) You may treat your different contractors of service for a given mode as separate groups when each is responsible for sampling and collecting data.
(2) You may treat your operational divisions as separate groups if each is responsible for sampling and collecting data.
(3) You may treat your commuter vanpool separately from your other vanpool services.