62 lines
2.1 KiB
R
62 lines
2.1 KiB
R
#
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# This code verified some oddities about model frames
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#
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options(contrasts=c("contr.treatment", "contr.poly")) # clean slate
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tdata <- data.frame(y=1:10, x1=letters[c(1,2,3,1,2,3,1,2,3,1)],
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x2=LETTERS[c(1,2,3,4,1,2,3,4,1,2)],
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stringsAsFactors=TRUE)
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tdata$x3 <- as.character(tdata$x1)
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fit1 <- lm(y ~ x1 + x2, tdata, x=TRUE)
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m1 <- fit1$model
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t1 <- terms(fit1)
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# Lesson 1: xlev is ignored when the variable already has levels
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# as an attribute
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temp <- list(x1=c("a", "c", "b"), x2=LETTERS[4:1])
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x2 <- model.matrix(t1, m1, xlev=temp)
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x3 <- model.matrix(t1, tdata, xlev=temp)
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all.equal(x2, fit1$x)
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!is.logical(all.equal(x2, x3)) # x2 and x3 do not agree
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attributes(m1$x1)
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# Lesson 2: character variables do not have their levels
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# remembered as attributes in the model frame, but these are
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# found in fit$xlevels and fit$contrasts.
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# However, the xlev argument is still ignored for a model frame!
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fit2 <- lm(y ~ x3 + x2, tdata, x=TRUE)
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m2 <- fit2$model
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x3 <- model.matrix(terms(fit2), m2, xlev=fit2$xlevels)
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x4 <- model.matrix(terms(fit2), m2, xlev=list(x3=letters[3:1]))
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x5 <- model.matrix(terms(fit2), tdata, xlev=list(x3=letters[3:1]))
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all.equal(fit2$x, x3)
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all.equal(x3, x4)
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all.equal(x3, x5) # FALSE
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# Lesson 3: contrasts.arg is relevant, even when the model frame
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# has a saved contrast
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ctemp <- list(x1="contr.SAS", x2= contr.helmert(LETTERS[1:4]))
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x4 <- model.matrix(t1, m1, contrasts.arg=ctemp) # no saved contrast
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fit3 <- lm(y ~ x1 + C(x2, contr.SAS), tdata)
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m3 <- fit3$model
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attr(m3[[3]], 'contr') # the contrast is saved
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c2 <- ctemp
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names(c2) <- names(fit3$contrasts)
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x5 <- model.matrix(terms(fit3), m3, contrasts.arg=c2)
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all.equal(x4, x5, check.attributes=FALSE)
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# Lesson 4: and this holds for a character variable as well
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fit4 <- lm(y ~ x3 + C(x2, contr.SAS), tdata, x=TRUE)
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c4 <- ctemp
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names(c4) <- names(fit4$contrasts)
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x6 <- model.matrix(terms(fit4), fit4$model, xlev=fit4$xlevels, contrasts.arg=c4)
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x7 <- model.matrix(terms(fit4), fit4$model, contrasts.arg=c4)
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all.equal(x7, x6)
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all.equal(x7, x5, check.attributes=FALSE)
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