thank you for thequick reply. i've coded the variable using SPSS. the steps were:  1. use "aggregate" in order to create a time-invaraiant baseline variable (the first time point) 2. then creating the centered variable by substracting this variable from the origincal variable.  looking at this variable seems to look OK (i've attached a screenshot showing my columns)  and i've attached the SAS output    Noa     ... View more