I'm working with longitudinal panel data and running models using the mixed command. In prepping the data, I excluded individuals who did not complete at least two time points of data collection (each person should've done 2 or 3 time points, not 0 or 1). Further, I thought I cleaned so subjs missing the covariates of interest were also dropped.

However, when I run my mixed model I get:
Code:
Number of observations = 7,499 
Number of groups = 3,290 

Obs per group: 
min = 1 
avg = 2.3 
max = 3
I would've expected it to be 3,290 "groups" (aka subjects) but then min of 2 observations per group (per person) and max of 3.

Is there a simple way I can identify (in this relatively large dataset) for which subjects only 1 of the time points was being included in the model?