use of formula in survey analysis with replicated weights

Le vendredi 28 juin 2013 ? 17:44 +0200, LE TERTRE Alain a ?crit :
Hi there,
I would like to use a formula inside a call to withReplicates in a survey analysis.
If the initial call with formula expressed inside the function works as expected, defining the formula outside gives an error message.
See example below, adapted from survey:withReplicates help page.

library(survey)
library(quantreg)

data(api)
## one-stage cluster sample
dclus1<-svydesign( id=~dnum, weights=~pw, data=apiclus1, fpc=~fpc)
## convert to bootstrap
bclus1<-as.svrepdesign( dclus1, type="bootstrap", replicates=100)

## median regression
withReplicates( bclus1, quote( coef( rq( api00~api99, tau=0.5, weights=.weights))))
               theta     SE
(Intercept) 87.78505 18.850
api99        0.91589  0.028

# Defining formula outside
Myformula <- as.formula( " api00~api99")
# Rerun the same analysis
withReplicates( bclus1, quote( coef( rq( formula= Myformula, tau=0.5, weights=.weights))))
Erreur dans eval(expr, envir, enclos) : objet 'api00' introuvable

# I suspect the evaluation not done in the right environment. 
#If you specify with data option in rq, the initial dataframe, formula is then correctly evaluated but .weights are not found.

withReplicates( bclus1, quote( coef( rq( formula= Myformula, tau=0.5, weights=.weights, data=apiclus1 ))))
Erreur dans eval(expr, envir, enclos) : objet '.weights' introuvable

Any help greatly appreciated
Here is a workaround:
Myformula <- "api00 ~ api99"

withReplicates(bclus1, quote(coef(rq(formula(Myformula), tau=0.5, weights=.weights))))

This solution makes sure the formula uses the environment where the
weights are available. If you call as.formula() from outside the
function, it will use the global environment. If you pass a character
string, it will be converted to a formula object deep in a function and
will thus use an environment where the weights are not be available
either.

Note that the same problem happens when using lm().

Regards
O__ ---- Alain Le Tertre
 c/ /'_ --- Institut de Veille Sanitaire (InVS)/ D?partement Sant? Environnement
(*) \(*) -- Responsable de l'unit? Statistiques & Outils
~~~~~~~~~~ - 12 rue du val d'Osne
94415 Saint Maurice cedex FRANCE
Voice: 33 1 41 79 67 62 Fax: 33 1 41 79 67 68
email: a.letertre at invs.sante.fr

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use of formula in survey analysis with replicated weights

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