Expand terms with '||' notation into separate '|' terms

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Description

From the right hand side of a formula for a mixed-effects model, expand terms with the double vertical bar operator into separate, independent random effect terms.

Usage

1

Arguments

term

a mixed-model formula

Value

the modified term

Note

Because || works at the level of formula parsing, it has no way of knowing whether a variable is a factor. It just takes the terms within a random-effects term and literally splits them into the intercept and separate no-intercept terms, e.g. (1+x+y|f) would be split into (1|f) + (0+x|f) + (0+y|f). However, || will fail to break up factors into separate terms; the dummy function can be useful in this case, although it is not as convenient as ||.

See Also

formula, model.frame, model.matrix, dummy.

Other utilities: mkRespMod, mkReTrms, nlformula, nobars, subbars

Examples

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  m <- ~ x + (x || g)
  expandDoubleVerts(m)
  set.seed(101)
  dd <- expand.grid(f=factor(letters[1:3]),g=factor(1:200),rep=1:3)
  dd$y <- simulate(~f + (1|g) + (0+dummy(f,"b")|g) + (0+dummy(f,"c")|g),
          newdata=dd,
          newparams=list(beta=rep(0,3),
                         theta=c(1,2,1),
                         sigma=1),
          family=gaussian)[[1]]
  m1 <- lmer(y~f+(f|g),data=dd)
  VarCorr(m1)
  m2 <- lmer(y~f+(1|g) + (0+dummy(f,"b")|g) + (0+dummy(f,"c")|g),
               data=dd)
  VarCorr(m2)

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