1 Performing variable selection

Forward selection, backward elimination, and branch and bound selection can be done using VariableSelection().
VariableSelection() can accept either a BranchGLM object or a formula along with the data and the desired family and link to perform the variable selection.
Available metrics are AIC, BIC and HQIC, which are used to compare models and to select the best models.
VariableSelection() returns some information about the search, more detailed information about the best models can be seen by using the summary() function.
Note that VariableSelection() will properly handle interaction terms and categorical variables.
keep can also be specified if any set of variables are desired to be kept in every model.

1.1 Metrics

The 3 different metrics available for comparing models are the following
- Akaike information criterion (AIC), which typically results in models that are useful for prediction
  - \(AIC = -2logLik + 2 \times p\)
- Bayesian information criterion (BIC), which results in models that are more parsimonious than those selected by AIC
  - \(BIC = -2logLik + \log{(n)} \times p\)
- Hannan-Quinn information criterion (HQIC), which is in the middle of AIC and BIC
  - \(HQIC = -2logLik + 2 * \log({\log{(n)})} \times p\)

1.2 Stepwise methods

Forward selection and backward elimination are both stepwise variable selection methods.
They are not guaranteed to find the best model or even a good model, but they are very fast.
Forward selection is recommended if the number of variables is greater than the number of observations or if many of the larger models don’t converge.
These methods will only return 1 best model.
Parallel computation can be used for the methods, but is generally only necessary for large datasets.

1.2.1 Forward selection example

# Loading BranchGLM package
library(BranchGLM)

# Fitting gamma regression model
cars <- mtcars

# Fitting gamma regression with inverse link
GammaFit <- BranchGLM(mpg ~ ., data = cars, family = "gamma", link = "inverse")

# Forward selection with mtcars
forwardVS <- VariableSelection(GammaFit, type = "forward")
forwardVS
#> Variable Selection Info:
#> ------------------------
#> Variables were selected using forward selection with AIC
#> The best value of AIC obtained was 142.2
#> Number of models fit: 27
#> Variables that were kept in each model:  (Intercept)
#> Order the variables were added to the model:
#> 
#> 1). wt
#> 2). hp

## Getting final model
fit(forwardVS, which = 1)
#> Results from gamma regression with inverse link function 
#> Using the formula mpg ~ hp + wt
#> 
#>              Estimate SE  t p.values
#> (Intercept) 8.923e-03 NA NA       NA
#> hp          8.887e-05 NA NA       NA
#> wt          9.826e-03 NA NA       NA
#> 
#> Dispersion parameter taken to be 0.0104
#> 32 observations used to fit model
#> (0 observations removed due to missingness)
#> 
#> Residual Deviance: 0.33 on 29 degrees of freedom
#> AIC: 142.2
#> Algorithm converged in 3 iterations using Fisher's scoring

1.2.2 Backward elimination example

# Backward elimination with mtcars
backwardVS <- VariableSelection(GammaFit, type = "backward")
backwardVS
#> Variable Selection Info:
#> ------------------------
#> Variables were selected using backward elimination with AIC
#> The best value of AIC obtained was 141.9
#> Number of models fit: 49
#> Variables that were kept in each model:  (Intercept)
#> Order the variables were removed from the model:
#> 
#> 1). vs
#> 2). drat
#> 3). am
#> 4). disp
#> 5). carb
#> 6). cyl

## Getting final model
fit(backwardVS, which = 1)
#> Results from gamma regression with inverse link function 
#> Using the formula mpg ~ hp + wt + qsec + gear
#> 
#>               Estimate SE  t p.values
#> (Intercept)  4.691e-02 NA NA       NA
#> hp           6.284e-05 NA NA       NA
#> wt           9.485e-03 NA NA       NA
#> qsec        -1.299e-03 NA NA       NA
#> gear        -2.662e-03 NA NA       NA
#> 
#> Dispersion parameter taken to be 0.0091
#> 32 observations used to fit model
#> (0 observations removed due to missingness)
#> 
#> Residual Deviance: 0.29 on 27 degrees of freedom
#> AIC: 141.9
#> Algorithm converged in 3 iterations using Fisher's scoring

1.3 Branch and bound

The branch and bound methods can be much slower than the stepwise methods, but they are guaranteed to find the best models.
The branch and bound methods are typically much faster than an exhaustive search and can also be made even faster if parallel computation is used.

1.3.1 Branch and bound example

If showprogress is true, then progress of the branch and bound algorithm will be reported occasionally.
Parallel computation can be used with these methods and can lead to very large speedups.

# Branch and bound with mtcars
VS <- VariableSelection(GammaFit, type = "branch and bound", showprogress = FALSE)
VS
#> Variable Selection Info:
#> ------------------------
#> Variables were selected using branch and bound selection with AIC
#> The best value of AIC obtained was 141.9
#> Number of models fit: 63
#> Variables that were kept in each model:  (Intercept)

## Getting final model
fit(VS, which = 1)
#> Results from gamma regression with inverse link function 
#> Using the formula mpg ~ hp + wt + qsec + gear
#> 
#>               Estimate SE  t p.values
#> (Intercept)  4.691e-02 NA NA       NA
#> hp           6.284e-05 NA NA       NA
#> wt           9.485e-03 NA NA       NA
#> qsec        -1.299e-03 NA NA       NA
#> gear        -2.662e-03 NA NA       NA
#> 
#> Dispersion parameter taken to be 0.0091
#> 32 observations used to fit model
#> (0 observations removed due to missingness)
#> 
#> Residual Deviance: 0.29 on 27 degrees of freedom
#> AIC: 141.9
#> Algorithm converged in 3 iterations using Fisher's scoring

A formula with the data and the necessary BranchGLM fitting information can also be used instead of supplying a BranchGLM object.

# Can also use a formula and data
formulaVS <- VariableSelection(mpg ~ . ,data = cars, family = "gamma", 
                               link = "inverse", type = "branch and bound",
                               showprogress = FALSE, metric = "AIC")
formulaVS
#> Variable Selection Info:
#> ------------------------
#> Variables were selected using branch and bound selection with AIC
#> The best value of AIC obtained was 141.9
#> Number of models fit: 63
#> Variables that were kept in each model:  (Intercept)

## Getting final model
fit(formulaVS, which = 1)
#> Results from gamma regression with inverse link function 
#> Using the formula mpg ~ hp + wt + qsec + gear
#> 
#>               Estimate SE  t p.values
#> (Intercept)  4.691e-02 NA NA       NA
#> hp           6.284e-05 NA NA       NA
#> wt           9.485e-03 NA NA       NA
#> qsec        -1.299e-03 NA NA       NA
#> gear        -2.662e-03 NA NA       NA
#> 
#> Dispersion parameter taken to be 0.0091
#> 32 observations used to fit model
#> (0 observations removed due to missingness)
#> 
#> Residual Deviance: 0.29 on 27 degrees of freedom
#> AIC: 141.9
#> Algorithm converged in 3 iterations using Fisher's scoring

1.3.2 Using bestmodels

The bestmodels argument can be used to find the top k models according to the metric.

# Finding top 10 models
formulaVS <- VariableSelection(mpg ~ . ,data = cars, family = "gamma", 
                               link = "inverse", type = "branch and bound",
                               showprogress = FALSE, metric = "AIC", 
                               bestmodels = 10)
formulaVS
#> Variable Selection Info:
#> ------------------------
#> Variables were selected using branch and bound selection with AIC
#> Found the top 10 models
#> The range of AIC values for the top 10 models is (141.9, 143.59)
#> Number of models fit: 122
#> Variables that were kept in each model:  (Intercept)

## Plotting results
plot(formulaVS, type = "b")


## Getting best model
fit(formulaVS, which = 1)
#> Results from gamma regression with inverse link function 
#> Using the formula mpg ~ hp + wt + qsec + gear
#> 
#>               Estimate SE  t p.values
#> (Intercept)  4.691e-02 NA NA       NA
#> hp           6.284e-05 NA NA       NA
#> wt           9.485e-03 NA NA       NA
#> qsec        -1.299e-03 NA NA       NA
#> gear        -2.662e-03 NA NA       NA
#> 
#> Dispersion parameter taken to be 0.0091
#> 32 observations used to fit model
#> (0 observations removed due to missingness)
#> 
#> Residual Deviance: 0.29 on 27 degrees of freedom
#> AIC: 141.9
#> Algorithm converged in 3 iterations using Fisher's scoring

1.3.3 Using cutoff

The cutoff argument can be used to find all models that have a metric value that is within cutoff of the minimum metric value found.

# Finding all models with an AIC within 2 of the best model
formulaVS <- VariableSelection(mpg ~ . ,data = cars, family = "gamma", 
                               link = "inverse", type = "branch and bound",
                               showprogress = FALSE, metric = "AIC", 
                               cutoff = 2)
formulaVS
#> Variable Selection Info:
#> ------------------------
#> Variables were selected using branch and bound selection with AIC
#> Found the top 16 models
#> The range of AIC values for the top 16 models is (141.9, 143.9)
#> Number of models fit: 116
#> Variables that were kept in each model:  (Intercept)

## Plotting results
plot(formulaVS, type = "b")

1.4 Using keep

Specifying variables via keep will ensure that those variables are kept through the selection process.

# Example of using keep
keepVS <- VariableSelection(mpg ~ . ,data = cars, family = "gamma", 
                               link = "inverse", type = "branch and bound",
                               keep = c("hp", "cyl"), metric = "AIC",
                               showprogress = FALSE, bestmodels = 10)
keepVS
#> Variable Selection Info:
#> ------------------------
#> Variables were selected using branch and bound selection with AIC
#> Found the top 10 models
#> The range of AIC values for the top 10 models is (143.17, 145.24)
#> Number of models fit: 55
#> Variables that were kept in each model:  (Intercept), hp, cyl

## Getting summary and plotting results
plot(keepVS, type = "b")


## Getting final model
fit(keepVS, which = 1)
#> Results from gamma regression with inverse link function 
#> Using the formula mpg ~ cyl + hp + wt + qsec + gear
#> 
#>               Estimate SE  t p.values
#> (Intercept)  6.464e-02 NA NA       NA
#> cyl         -1.412e-03 NA NA       NA
#> hp           7.523e-05 NA NA       NA
#> wt           1.037e-02 NA NA       NA
#> qsec        -1.816e-03 NA NA       NA
#> gear        -3.861e-03 NA NA       NA
#> 
#> Dispersion parameter taken to be 0.0089
#> 32 observations used to fit model
#> (0 observations removed due to missingness)
#> 
#> Residual Deviance: 0.29 on 26 degrees of freedom
#> AIC: 143.17
#> Algorithm converged in 3 iterations using Fisher's scoring

1.5 Convergence issues

It is not recommended to use branch and bound if the upper models do not converge since it can make the algorithm very slow.
Sometimes when using backwards selection and all the upper models that are tested do not converge, no final model can be selected.
For these reasons, if there are convergence issues it is recommended to use forward selection.

VariableSelection Vignette

1 Performing variable selection

1.1 Metrics

1.2 Stepwise methods

1.2.1 Forward selection example

1.2.2 Backward elimination example

1.3 Branch and bound

1.3.1 Branch and bound example

1.3.2 Using bestmodels

1.3.3 Using cutoff

1.4 Using keep

1.5 Convergence issues