I have 3 waves of panel data. The dv is measured at wave 3. There is one covariate (continuous data) of particular interest that i would like to consider it's impact across all three waves of data. That is, rather than use dummy variables for wave1, wave 2 and wave3 separately i was wondering if it is possible in some way use the data to determine the relationship between the patterns of covariate and the outcome variable. Rather then use something like a cluster analysis (which would profoundly decrease my sample size) is it reasonable for instance to estimate the mean and slope of each subject and model this with respect to the outcome variable. Is it possible to do this within 1 model or would I need to run 2 models.
You can always string out panel data into one long observation for analysis. That way you avoid the homoscedasticity problems often associated with repeated measures analyses.
You can look at the slope in one model (in GLM) using a TEST statement and coefficients that reflect the shape that you want to test (linear, here). This will only use complete observations.
You can address the complete observation challenge (which is sounds like is substantial) with PROC MI to get a better estimate of the behavior than simple linear interpolation (which is what would happen if you used the within-subject slope).