Hi,
Im trying to set up a long format data structure to use xtgee models. For each time variable (week) there are three tilt levels. The other variables eg sbp are measured at each tilt level for each assessment week.
When I try to xtset with id week this is not allowed because of repeat weeks (one for each tilt level). I wish to assess time trends in measurements within id across weeks and assess changes within individuals based upon tilt.
Clearly I have set up an incorrect structure for xtset or this is not possible?
Here is an excerpt of the data structure

id week tilt sbp
1 20 10 119
1 20 15 114
1 20 0 117
1 30 15 .
1 30 0 121
1 30 10 113
2 20 0 .
2 20 15 93
2 20 10 106
2 30 15 123
2 30 0 123

any advice would be greatly appreciated
yours
Richard Hiscock