Thanks, it works well !   But it seems to have a mistake. I have to had the object produce by zelig() in the call() function. So the code is this :     call1 <- call2 <- call("setx", z.out.1, fn = NULL) Merci, François Maurice, B. Sc., A. Stat. Candidat à la maîtrise Département de sociologie Université de Montréal De : "Owen, Matt" &lt;mowen(a)iq.harvard.edu&gt; À : Francois Maurice &lt;maurice.francois(a)ymail.com&gt;om>; Zelig mailing list &lt;zelig(a)lists.gking.harvard.edu&gt; Envoyé le : Mardi 6 Septembre 2011 22h46 Objet : RE: [zelig] Problem in specifying a proxy variable in setx() The operation that you are attempting to do is:   x.out.0.1 <- setx(z.out.1, fn=NULL, TREAT=0)   x.out.1.1 <- setx(z.out.1, fn=NULL, TREAT=1) The reason why your statement does not work is more of an issue with programming in general. That is, when the function setx is evaluated, the statement:   tx.var = 0 is being understood that you are setting a parameter (tx.var) to whatever is on the right-hand-side of the equal sign. To do what you are trying to do (in the same manner as you are trying to do it) requires something like:   call1 <- call2 <- call("setx", fn = NULL)   call1[[tx.var]] <- 0   call2[[tx.var]] <- 1   x.out.0.1 <- eval(call1)   x.out.1.1 <- eval(call2) Why this is the case has to do with how R interprets what you are writing in your script. That is, imagine the scenario where the formula (causal.model) was:   causal.model <- QASBAT ~ TREAT + T + T2 + T3 + tag(T | ID.factor) + tx.var Note that tx.var now is specified as an explanatory variable. In general, writing out:   x.out.0.1 <- setx(z.out.1, fn=NULL, TREAT=0)   x.out.1.1 <- setx(z.out.1, fn=NULL, TREAT=1) is typically the better option because of its readability. ________________________________________ From: owner-zelig_at_lists_gking_harvard_edu(a)mail.hmdc.harvard.edu [owner-zelig_at_lists_gking_harvard_edu(a)mail.hmdc.harvard.edu] On Behalf Of Francois Maurice [maurice.francois(a)ymail.com] Sent: Tuesday, September 06, 2011 10:19 PM To: Zelig mailing list Subject: [zelig] Problem in specifying a proxy variable in setx() Hi, I'm trying to do elegant coding, but I have trouble with setx(). I defined two objects, tx.var, wich contains the name of the treatment variable, and causal.model, which contains the model used in zelig() (see the code below). Everything works fine except in setx(). When I specify tx.var instead of TREAT, which is the name of the treatment variable, sim() produce zero effect. But when I specify TREAT, sim() produces a quantity. Is there something I can do to correct this ? tx.var <- c("TREAT") causal.model <- QASBAT ~ TREAT + T + T2 + T3 + tag(T | ID.factor) (some other codes here) z.out.1 <- zelig(formula= as.formula(causal.model), data=matched.1.mtch.long,  model="ls.mixed") x.out.0.1 <- setx(z.out.1, fn=NULL, tx.var=0) x.out.1.1 <- setx(z.out.1, fn=NULL, tx.var=1) s.out.1 <- sim(z.out.1, x=x.out.0.1, x1=x.out.1.1) Merci, François Maurice, B. Sc., A. Stat. Candidat à la maîtrise Département de sociologie Université de Montréal - Zelig Mailing List, served by Harvard-MIT Data Center Send messages: zelig(a)lists.gking.harvard.edu [un]subscribe Options: http://lists.gking.harvard.edu/?info=zelig Zelig program information: http://gking.harvard.edu/zelig/