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Re: st: RE: RE: dummy dropped in xtgee
--- Jessica Kiefte-de Jong <[email protected]> wrote:
> But how can I know if there is an effect of the intervention at
> 6 months? For all the other time periods I'll use interaction
> terms (e.g. Dtime12*intevention) but I cannot perform that for the
> 6 months dummy.
Lets simplify the problem and say that you have only two time periods
(6 and 12 months) and thus only one time dummy. Your equation would
look something like this:
y = b0 + b1 intervention + b2 Dtime12 + b3 Dtime12*intervention
The predicted value of response/explanatory/y variable for the
intervention group at time 6 is:
b0 + b1 * 1 + b2 * 0 + b3 * 0 * 1 = b0 + b1
The predicted value of response/explanatory/y variable for control
group at time 6 is:
b0 + b1 * 0 + b2 * 0 + b3 * 0 * 0 = b0
The difference between intervention and control group at time 6 is:
b0 + b1 - b0 = b1 (the main effect of intervention)
Similarly for time 12:
The predicted value of response/explanatory/y variable for the
intervention group at time 12 is:
b0 + b1 * 1 + b2 * 1 + b3 * 1 * 1 = b0 + b1 + b2 + b3
The predicted value of response/explanatory/y variable for control
group at time 6 is:
b0 + b1 * 0 + b2 * 1 + b3 * 1 * 0 = b0 + b2
The difference between intervention and control group at time 6 is:
b0 + b1 + b2 + b3 - (b0 + b2) = b1 + b3 (the main effect of
intervention)
Hope this helps,
Maarten
-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands
visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434
+31 20 5986715
http://home.fsw.vu.nl/m.buis/
-----------------------------------------
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