remef

remef is an R package designed for the removal of statistical effects.

The aim of remef is to provide tools for extracting partial effects from data.

Mixed-effects models include both fixed and random effects. The dependent variable is expressed as a combination of fixed and random effects and an error term. remef is a practical tool for removing any combination of fixed and random effects from the data. The removal of effects is very helpful for understanding and visualizing certain effects in highly complex statistical models. Currently, the package works with objects generated by the package lme4.

Key features:

• removes fixed effects and random variance from a dependent variable.

• optionally, remef takes into account associated lower-order or higher-order effects when removing partial effects (this is important for statistical models including interactions).

• finds model coefficients for a given term (e.g., for factors and polynomial contrasts) and vice versa.

Installation

To get the current development version from github, you can use the package devtools:

# install.packages("devtools")
devtools::install_github("hohenstein/remef")

Examples

For the examples, we create a simple linear mixed effects model. It is one of the examples of the lmer function. The model consists of an intercept (1), the fixed effect Days and the corresponding variance components (random intercept and slope) for the random factor Subject.

library(lme4)
fm1 <- lmer(Reaction ~ 1 + Days + (1 + Days | Subject), sleepstudy)

Of course, we have to load the remef package:

library(remef)

Remove a fixed effect

The following commands are a demonstration of how the remef package can be used to remove the influence of certain effects from a dependent variable (Reaction).

Remove the fixed effect of Days:

p_fm1_1 <- remef(fm1, fix = "Days", keep.intercept = TRUE)

Since keep.intercept = TRUE is the default, we can shorten this command to

p_fm1_2 <- remef(fm1, fix = "Days")

We can also used the index of the coefficient name "Days" instead of the name. The intercept corresponds to index 1, Days corresponds to index 2.

p_fm1_3 <- remef(fm1, fix = 2)

Remove a variance component (random effect)

In this example, the random slope of Days should be removed:

p_fm1_4 <- remef(fm1, ran = list(Subject = "Days"))

Of course, we can replace the character string "Days" with its index. Note that the index corresponds to the order of variance components for the random factor Subject and is not related to the order of fixed-effect coefficients.

p_fm1_5 <- remef(fm1, ran = list(Subject = 2))

Remove the intercept

To remove the intercept, we can use keep.intercept = FALSE.

p_fm1_6 <- remef(fm1, keep.intercept = FALSE)

The above command will remove the intercept and therefore center Reaction on the intercept.

Remove multiple fixed and random effects

If we want to remove both fixed effects and both random effects, we can use

p_fm1_7 <- remef(fm1, fix = "Days", ran = list(Subject = c(1, 2)), keep.intercept = FALSE)

Since this command will remove all fixed and random effects, the result is equal to the residuals of the model.

If we want to remove all random effects from a model, we can pass the character string "all" to the parameter ran. This is a simple alternative to a list including all random effects, particularly for complex models with multiple random factors.

p_fm1_8 <- remef(fm1, ran = "all")

Keeping effects: the function keepef

If we want to remove a large proportion of fixed or random effects, it might be easier to specify the effects that should be kept instead. Here, the function keepef can be used. In contrast to remef, keepef keeps the specified effects but removes all the remaining ones.

# the following commands are euqivalent
p_fm1_9 <- remef(fm1, fix = "Days", ran = "all")
p_fm1_10 <- keepef(fm1)

The commands above remove all effects except the intercept. Note that keep.intercept defaults to TRUE for keepef too. If we want to remove all effects including the intercept with keepef, we have to use

p_fm1_11 <- keepef(fm1, keep.intercept = FALSE)

The values obtained with the command above are equal to the model residuals.

More examples

For more examples, have a look at the documentations of remef and keepef.

?remef
?keepef

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Important information for users of the old standalone remef function

Before the launch of the remef package, the function remef was available from the Potsdam Mind Research Repository only. The latest version of the standalone function is 0.6.10. This function will not be developed further, but remain available. Please use its successor, the remef package.

If you have already been working with the old standalone function, note that there are some syntactic differences to the new version in the package:

• A mixture of coefficient names (character strings) and numeric indices (e.g., c(1, "factorA")) for fix or ran is no longer valid. In the current version, you can use vectors of either coefficient names (character strings) or numeric indices corresponding to the model coefficients.

• The parameter keep, that was used to specify whether effect should be removed or kept, is no longer part of the remef function. In the package, there is a function remef for removing effects and a function keepef for keeping effects.

• There is a new parameter keep.intercept that defaults to TRUE. It indicates whether the intercept should be kept (if keep.intercept = TRUE) or be removed (if keep.intercept = FALSE). It is not allowed to use the intercept in the parameter fix, e.g., "(Intercept)" or 1. For example, if you want to remove a fixed effect "factorA" and the intercept, use the parameters fix = "factorA" and keep.intercept = FALSE.

• The list ran must be a named list now. In the old remef function, the order of its list elements was the same as the order of random effects in the model. In the new remef package, the names of ran correspond to the names of the random factors. An example list ran is list(Subject = c(1, 3), Item = c(2, 3, 4)).

• In the old remef function, the elements of the list ran were numeric index vectors corresponding to variance components (random intercept/slopes). Now, you can use character vectors including names of the variance components too. You can use both numeric index vectors and character vectors in ran, e.g., list(Subject = c("(Intercept)", "Factor2"), Item = c(2, 3, 4))). For a model fit object model, the command ranef_labels(model) returns a list containing the names of all variance components of all random factors.

• There is a new (logical) parameter inverse that indicates whether the inverse of the link function is applied. It replaces the old family parameter that accepted two values, gaussian and binomial. In the new remef package, the family of the model is detected automatically. The parameter inverse is important for generalised linear mixed-effects models, created with lme4::glmer, only.