Center variables in fitted regression models

center_mod (previously known as center_lm) takes fitted regression models and mean-centers the continuous variables in the model to aid interpretation, especially in the case of models with interactions. It is a wrapper to scale_mod.

Usage

center_mod(
  model,
  binary.inputs = "0/1",
  center.response = FALSE,
  data = NULL,
  apply.weighted.contrasts = getOption("jtools-weighted.contrasts", FALSE),
  ...
)

Arguments

model: A regression model of type lm, glm, or svyglm; others may work as well but have not been tested.
binary.inputs: Options for binary variables. Default is 0/1; 0/1 keeps original scale; -0.5,0.5 rescales 0 as -0.5 and 1 as 0.5; center subtracts the mean; and full treats them like other continuous variables.
center.response: Should the response variable also be centered? Default is FALSE.
data: If you provide the data used to fit the model here, that data frame is used to re-fit the model instead of the stats::model.frame() of the model. This is particularly useful if you have variable transformations or polynomial terms specified in the formula.
apply.weighted.contrasts: Factor variables cannot be scaled, but you can set the contrasts such that the intercept in a regression model will reflect the true mean (assuming all other variables are centered). If set to TRUE, the argument will apply weighted effects coding to all factors. This is similar to the R default effects coding, but weights according to how many observations are at each level. An adapted version of contr.wec() from the wec package is used to do this. See that package's documentation and/or Grotenhuis et al. (2016) for more info.
...: Arguments passed on to gscale().

Value

The functions returns a lm or glm object, inheriting from whichever class was supplied.

Details

This function will mean-center all continuous variables in a regression model for ease of interpretation, especially for those models that have interaction terms. The mean for svyglm objects is calculated using svymean, so reflects the survey-weighted mean. The weight variables in svyglm are not centered, nor are they in other lm family models.

This function re-estimates the model, so for large models one should expect a runtime equal to the first run.

References

Bauer, D. J., & Curran, P. J. (2005). Probing interactions in fixed and multilevel regression: Inferential and graphical techniques. Multivariate Behavioral Research, 40(3), 373-400.

Cohen, J., Cohen, P., West, S. G., & Aiken, L. S. (2003). Applied multiple regression/correlation analyses for the behavioral sciences (3rd ed.). Mahwah, NJ: Lawrence Erlbaum Associates, Inc.

Author

Jacob Long jacob.long@sc.edu

Examples


fit <- lm(formula = Murder ~ Income * Illiteracy,
          data = as.data.frame(state.x77))
fit_center <- center_mod(fit)

# With weights
fitw <- lm(formula = Murder ~ Income * Illiteracy,
           data = as.data.frame(state.x77),
           weights = Population)
fitw_center <- center_mod(fitw)

# With svyglm
if (requireNamespace("survey")) {
library(survey)
data(api)
dstrat <- svydesign(id = ~1, strata = ~stype, weights = ~pw,
                    data = apistrat, fpc =~ fpc)
regmodel <- svyglm(api00 ~ ell * meals, design = dstrat)
regmodel_center <- center_mod(regmodel)
}
#> Loading required namespace: survey
#> Loading required package: grid
#> Loading required package: Matrix
#> Loading required package: survival
#> 
#> Attaching package: ‘survey’
#> The following object is masked from ‘package:graphics’:
#> 
#>     dotchart