Dear R-help mailing list, Kindly help me out with this problem: I have a dataset that is in the format below, ID time Y Age 1 0 195 23.1 1 2 204 23.3 1 4 202 23.5 2 0 170 22.0 2 3 234 22.2 3 0 208 24.4 3 2 194 24 .7 3 3 204 24.9 I wish to remove all the measurements at time point 0 and convert them to a baseline variable as follows; ID time Y Age baseline 1 2 204 23.3 195 1 4 202 23.5 195 2 3 234 22.2 170 3 2 194 24 .7 208 3 3 204 24.9 208 Thanks. regards John
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