I need to know the correct test to use.
I have three groups of people who underwent a procedure.  I have a lab
value before and after the procedure.  The three groups are tertiles
of the length of time of the procedure. (o minutes, 0-30 minutes, and
>30 minutes).  I don't think the lab value is altered by the
procedure.  I have the pre-procedure and post-procedure lab value for
every patient.  I want to show that the change in the lab value is the
SAME between all three groups.  In other words, I have three groups of
people who underwent a certian procedure.  For every patient I have a
lab value (say, X) before and after the procedure.  Thus x(before)
minus x(after) equals delta-X, aka change in X.  I want to see if the
change in X, or delta X, differs between the three groups.
Do I use ANOVA?  What kind?  t-test?