Also known as feature selection in machine learning, the goal of variable selection is to identify a subset of predictors to simplify models. This can benefit model interpretation, shorten fitting time, and improve generalization (by reducing overfitting).
There are many different methods. The appropriate method for a given problem will depend on the model type, the data, the objective, and the theoretical rationale.
The parameters package implements a helper that will
automatically pick a method deemed appropriate for the
provided model, run the variables selection and return the
optimal formula, which you can then re-use to update
the model.
Simple linear regression
Fit a powerful model
If you are familiar with R and the formula interface, you know of the
possibility of including a dot (.) in the formula,
signifying “all the remaining variables”. Curiously, few are aware of
the possibility of additionally easily adding “all the interaction
terms”. This can be achieved using the .*. notation.
Let’s try that with the linear regression predicting
Sepal.Length with the iris
dataset, included by default in R.
model <- lm(Sepal.Length ~ . * ., data = iris)
summary(model)
#>
#> Call:
#> lm(formula = Sepal.Length ~ . * ., data = iris)
#>
#> Residuals:
#> Min 1Q Median 3Q Max
#> -0.726 -0.210 0.014 0.213 0.713
#>
#> Coefficients:
#> Estimate Std. Error t value Pr(>|t|)
#> (Intercept) 1.6998 1.0576 1.61 0.1103
#> Sepal.Width 0.8301 0.3047 2.72 0.0073 **
#> Petal.Length 0.3178 0.7852 0.40 0.6863
#> Petal.Width 2.5827 1.5874 1.63 0.1061
#> Speciesversicolor -2.7193 1.6201 -1.68 0.0956 .
#> Speciesvirginica -6.1704 3.2045 -1.93 0.0563 .
#> Sepal.Width:Petal.Length -0.0149 0.2185 -0.07 0.9458
#> Sepal.Width:Petal.Width -0.6112 0.4536 -1.35 0.1801
#> Sepal.Width:Speciesversicolor 0.4300 0.6666 0.65 0.5200
#> Sepal.Width:Speciesvirginica 0.8288 1.0031 0.83 0.4101
#> Petal.Length:Petal.Width -0.1195 0.3300 -0.36 0.7178
#> Petal.Length:Speciesversicolor 0.7398 0.5166 1.43 0.1544
#> Petal.Length:Speciesvirginica 0.9034 0.6938 1.30 0.1951
#> Petal.Width:Speciesversicolor -1.0070 1.2454 -0.81 0.4202
#> Petal.Width:Speciesvirginica -0.2864 1.5065 -0.19 0.8495
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#>
#> Residual standard error: 0.3 on 135 degrees of freedom
#> Multiple R-squared: 0.882, Adjusted R-squared: 0.87
#> F-statistic: 72.1 on 14 and 135 DF, p-value: <2e-16Wow, that’s a lot of parameters! And almost none of them are significant!
Which is weird, considering that gorgeous R^2 of 0.882!
I wish I had that in my research!
Too many parameters?
As you might know, having a model that is too performant is not always a good thing. For instance, it can be a marker of overfitting: the model corresponds too closely to a particular set of data, and may therefore fail to predict future observations reliably. In multiple regressions, in can also fall under the Freedman’s paradox: some predictors that have actually no relation to the dependent variable being predicted will be spuriously found to be statistically significant.
Let’s run a few checks using the performance package:
library(performance)
check_normality(model)
#> OK: residuals appear as normally distributed (p = 0.612).
check_heteroscedasticity(model)
#> OK: Error variance appears to be homoscedastic (p = 0.118).
check_autocorrelation(model)
#> OK: Residuals appear to be independent and not autocorrelated (p = 0.574).
check_collinearity(model)
#> # Check for Multicollinearity
#>
#> High Correlation
#>
#> Term VIF VIF 95% CI Increased SE Tolerance
#> Sepal.Width 29.42 [ 22.31, 38.91] 5.42 0.03
#> Petal.Length 3204.78 [ 2414.74, 4253.41] 56.61 3.12e-04
#> Petal.Width 2442.10 [ 1840.11, 3241.14] 49.42 4.09e-04
#> Species 3.98e+05 [3.00e+05, 5.28e+05] 630.97 2.51e-06
#> Sepal.Width:Petal.Length 2183.98 [ 1645.63, 2898.55] 46.73 4.58e-04
#> Sepal.Width:Petal.Width 1866.48 [ 1406.42, 2477.15] 43.20 5.36e-04
#> Sepal.Width:Species 3.49e+05 [2.63e+05, 4.64e+05] 591.13 2.86e-06
#> Petal.Length:Petal.Width 4032.80 [ 3038.60, 5352.41] 63.50 2.48e-04
#> Petal.Length:Species 1.23e+06 [9.27e+05, 1.63e+06] 1109.09 8.13e-07
#> Petal.Width:Species 3.77e+05 [2.84e+05, 5.01e+05] 614.38 2.65e-06
#> Tolerance 95% CI
#> [0.03, 0.04]
#> [0.00, 0.00]
#> [0.00, 0.00]
#> [0.00, 0.00]
#> [0.00, 0.00]
#> [0.00, 0.00]
#> [0.00, 0.00]
#> [0.00, 0.00]
#> [0.00, 0.00]
#> [0.00, 0.00]The main issue of the model seems to be the high multicollinearity. This suggests that our model might not be able to give valid results about any individual predictor, nor tell which predictors are redundant with respect to others.
Parameters selection
Time to do some variables selection! This can be easily done using
the select_parameters() function in
parameters. It will automatically select
the best variables and update the model accordingly. One way of using
that is in a tidy pipeline (using %>%),
using this output to update a new model.
library(parameters)
lm(Sepal.Length ~ . * ., data = iris) |>
select_parameters() |>
summary()
#>
#> Call:
#> lm(formula = Sepal.Length ~ Sepal.Width + Petal.Length + Petal.Width +
#> Species + Sepal.Width:Petal.Width + Petal.Length:Species +
#> Petal.Width:Species, data = iris)
#>
#> Residuals:
#> Min 1Q Median 3Q Max
#> -0.7261 -0.2165 0.0021 0.2191 0.7439
#>
#> Coefficients:
#> Estimate Std. Error t value Pr(>|t|)
#> (Intercept) 2.090 0.528 3.95 0.00012 ***
#> Sepal.Width 0.734 0.130 5.66 8.3e-08 ***
#> Petal.Length 0.232 0.260 0.89 0.37310
#> Petal.Width 1.051 0.532 1.98 0.04993 *
#> Speciesversicolor -1.047 0.547 -1.92 0.05754 .
#> Speciesvirginica -2.682 0.638 -4.21 4.6e-05 ***
#> Sepal.Width:Petal.Width -0.232 0.103 -2.24 0.02667 *
#> Petal.Length:Speciesversicolor 0.660 0.298 2.22 0.02837 *
#> Petal.Length:Speciesvirginica 0.720 0.273 2.63 0.00941 **
#> Petal.Width:Speciesversicolor -1.112 0.550 -2.02 0.04528 *
#> Petal.Width:Speciesvirginica -0.499 0.460 -1.09 0.27934
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#>
#> Residual standard error: 0.3 on 139 degrees of freedom
#> Multiple R-squared: 0.88, Adjusted R-squared: 0.872
#> F-statistic: 102 on 10 and 139 DF, p-value: <2e-16That’s still a lot of parameters, but as you can see, almost all of them are now significant, and the R^2 did not change much.
Although appealing, please note that these automated selection methods are quite criticized, and should not be used in place of theoretical or hypothetical reasons (i.e., you should have a priori hypotheses about which parameters of your model you want to focus on).
Mixed models
For simple linear regressions as above, the selection is made using
the step() function (available in base R). This performs a
stepwise
selection. However, this procedures is not available for other types of
models, such as mixed models.
Mixed models
For mixed models (of class merMod), stepwise selection
is based on cAIC4::stepcAIC(). This step function only
searches the “best” model based on the random effects
structure, i.e. select_parameters() adds or excludes
random effects until the cAIC can’t be improved
further.
This is what our initial model looks like.
library(lme4)
data("qol_cancer")
# initial model
lmer(
QoL ~ time + phq4 + age + (1 + time | hospital / ID),
data = qol_cancer
) |>
summary()
#> Linear mixed model fit by REML ['lmerMod']
#> Formula: QoL ~ time + phq4 + age + (1 + time | hospital/ID)
#> Data: qol_cancer
#>
#> REML criterion at convergence: 4647
#>
#> Scaled residuals:
#> Min 1Q Median 3Q Max
#> -3.581 -0.393 0.082 0.507 2.505
#>
#> Random effects:
#> Groups Name Variance Std.Dev. Corr
#> ID:hospital (Intercept) 202.49 14.23
#> time 12.54 3.54 -0.72
#> hospital (Intercept) 8.47 2.91
#> time 1.27 1.13 -1.00
#> Residual 143.11 11.96
#> Number of obs: 564, groups: ID:hospital, 188; hospital, 2
#>
#> Fixed effects:
#> Estimate Std. Error t value
#> (Intercept) 70.6514 3.1279 22.59
#> time 1.4896 1.2194 1.22
#> phq4 -4.7687 0.3235 -14.74
#> age -0.0173 0.1889 -0.09
#>
#> Correlation of Fixed Effects:
#> (Intr) time phq4
#> time -0.954
#> phq4 0.014 -0.011
#> age -0.017 0.003 0.107
#> optimizer (nloptwrap) convergence code: 0 (OK)
#> boundary (singular) fit: see help('isSingular')This is the model selected by select_parameters().
Please notice the differences in the random effects structure between
the initial and the selected models:
## TODO: this is currently broken due to an issue in package cAIC4
# multiple models are checked, however, initial models
# already seems to be the best one...
lmer(
QoL ~ time + phq4 + age + (1 + time | hospital / ID),
data = qol_cancer
) |>
select_parameters() |>
summary()