Best way to tell if a formula contains a random effect?

user_8387242 0
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I have a list of formulas that I would like to fit in a loop using a function. Some of these formulas are random effects models and others are straightforward linear models. I want the function to detect whether the model contains a random effect and if so, use lmer() to fit the model. Otherwise, it should use lm(). Any suggestions on how to check this condition (other than converting the formula to a string and checking for parentheses)? At this stage, they have the same class so I can't just check that. I could also use error handling to catch when lmer() returns an error from a model without a random effect and reroute towards regular lm(), but this also seems unnecessarily messy.

Example below:

fit_models <- function(formula_list) {
  models <- list()
    for(ii in seq_along(formula_list)) {
      if(formula_list[[ii]] is lmer) { # Enter condition here
        print("lmer")
      } else {
        print("lm")
      }
  }
}

f1 <- formula(y ~ x)
f2 <- formula(y ~ 1 + x + (1 + x | z))
formulas <- c(f1, f2)

fit_models(formulas)
lme4 lm r function
3 Answers

I would say

length(lme4::findbars(f))>0

should reliably detect formulas containing a random-effects component (in the lme4 sense).

From the right hand side of a formula for a mixed-effects model, determine the pairs of expressions that are separated by the vertical bar operator.

This is (implicitly) the test that's done in the lme4 code, here ...

user_1902770

The symbols in formulas don't have inherent meanings. A function can reinterpret the symbols to mean whatever they like. So just because there is a "|", that doesn't mean necessarily that that's a formula that has a random effect. That's just how lmer chose to interpret that symbol.

Given that formulas are basically just ordered collections of unevaluated symbols, there's not much more you can do than a basic equality check for a symbol operating on just the formula itself. Rather than a strait up character conversion, you could use all.names. So something like

f2 <- formula(y ~ 1 + x + (1 + x | z))
all.names(f2)
#  [1] "~" "y" "+" "+" "x" "(" "|" "+" "x" "z"
"|" %in% all.names(f2)
# [1] TRUE

This won't be fooled if you have something like formula(`a|b` ~ x) where a|b is a (terrible) column name.

user_23720640

You can just convert the formula to a character and look for the pipe operator |:

f1 <- formula(y ~ x)
f2 <- formula(y ~ 1 + x + (1 + x | z))
formulas <- c(f1, f2)

sapply(formulas, function(x) any(grepl("\\|", as.character(x))))
#> [1] FALSE  TRUE
user_125003150
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