Each SIPP panel is divided up into four rotation groups and one rotation group is interviewed each month. This variable classifies each SIPP respondent into a particular rotation group and enables researhers to tell which month a particular respondent was asked the topical module questions. For example, if a person is in the first rotation group of the 1990 panel, his/her wave 1 interview covered January 1990-April 1990 and the wave 2 interview covered May 1990-August 1990. The wave 2 topical modules would have been asked in September 1990 at the time of the wave 2 interview. Schedules for interviewing rotation groups by panel can be found in the SIPP Users' Guide.