# Regression Analysis with Lasso Penalty In lslx: Semi-Confirmatory Structural Equation Modeling via Penalized Likelihood or Least Squares

```options(digits = 3)
options(width = 100)
```

In this example, we will show how to use `lslx` to conduct regression analysis with lasso penalty.

## Data Generation

The following code is used to generate data for regression analysis.

```set.seed(9487)
x <- matrix(rnorm(2000), 200, 10)
colnames(x) <- paste0("x", 1:10)
y <- matrix(rnorm(200), 200, 1)
data_reg <- data.frame(y, x)
```

The data set contains 200 observation on 10 covariates (`x1` - `x10`) and a response variable (`y`). By the construction of the data, the 10 covariates are not useful to predict the response. The data is stored in a `data.frame` named `data_reg`.

## Model Specification

Model specification in `lslx` is quite similar to that in `lavaan`. However, different operators and prefix are used to accommodate the presence of penalized parameters. In the following specification, `y` is predicted by `x1` - `x10`.

```model_reg <- "y <= x1 + x2 + x3 + x4
y <~ x5 + x6 + x7 + x8 + x9 + x10"
```

The operator `<=` means that the regression coefficients from the RHS variables to the LHS variables are freely estimated. On the other hand, the operator `<~` means that the regression coefficients from the RHS variables to the LHS variables are estimated with penalty. Details of model syntax can be found in the section of Model Syntax via `?lslx`. After version 0.6.3, `lslx` also support basic `lavaan` operators, including `=~`, `~`, and `~~`.

## Object Initialization

`lslx` is written as an `R6` class. Every time we conduct analysis with `lslx`, an `lslx` object must be initialized. The following code initializes an `lslx` object named `lslx_reg`.

```library(lslx)
lslx_reg <- lslx\$new(model = model_reg, data = data_reg)
```

Here, `lslx` is the object generator for `lslx` object and `\$new()` is the build-in method of `lslx` to generate a new `lslx` object. The initialization of `lslx` requires users to specify a model for model specification (argument `model`) and a data set to be fitted (argument `sample_data`). The data set must contain all the observed variables specified in the given model. In is also possible to initialize an `lslx` object via importing sample moments (see `vignette("structural-equation-modeling")`).

## Model Fitting

After an `lslx` object is initialized, method `\$fit()` can be used to fit the specified model into the given data.

```lslx_reg\$fit(penalty_method = "lasso", lambda_grid = seq(.00, .30, .01))
```

The fitting process requires users to specify the penalty method (argument `penalty_method`) and the considered penalty levels (argument `lambda_grid`). In this example, the `lasso` penalty is implemented on the lambda grid `seq(.00, .30, .01)`. All the fitting result will be stored in the `fitting` field of `lslx_reg`.

## Model Summarizing

Unlike traditional SEM analysis, `lslx` fit the model into data under all the penalty levels considered. To summarize the fitting result, a selector to determine an optimal penalty level must be specified. Available selectors can be found in the section of Penalty Level Selection via `?lslx`. The following code summarize the fitting result under the penalty level selected by Akaike information criterion (AIC).

```lslx_reg\$summarize(selector = "aic")
```

In this example, we can observe that all of the penalized coefficients are identified as zero, which is consistent with their population values. The `\$summarize()` method also shows the result of significance tests for the coefficients. In `lslx`, the default standard errors are calculated based on a sandwich formula whenever raw data is available. It is generally valid even when the model is misspecified and the data is not normal distributed. However, it may not be valid after selecting an optimal penalty level.

## Visualization

`lslx` provides four methods for visualizing the fitting results. The method `\$plot_numerical_condition()` shows the numerical condition under all the penalty levels. The following code plots the values of `n_iter_out` (number of iterations in outer loop), `objective_gradient_abs_max` (maximum of absolute value of gradient of objective function), and `objective_hessian_convexity` (minimum of univariate approximate hessian). The plot can be used to evaluate the quality of numerical optimization. `n_iter_out` shows that the algorithm converges quickly under all the penalty levels. `objective_gradient_abs_max` and `objective_hessian_convexity` indicate that the obtained coefficients are valid minimizers under all the penalty levels.

```lslx_reg\$plot_numerical_condition()
```

The method `\$plot_information_criterion()` shows the values of information criteria under all the penalty levels. The plot shows that an optimal value of lambda is any value larger than `0.15`.

```lslx_reg\$plot_information_criterion()
```

The method `\$plot_fit_index()` shows the values of fit indices under all the penalty levels.

```lslx_reg\$plot_fit_index()
```

The method `\$plot_coefficient()` shows the solution path of coefficients in the given block. The following code plots the solution paths of all coefficients in the block `y<-y`, which contains all the regression coefficients from observed variables to observed variables. We can see that all the regression coefficients become zero when the value of lambda is larger than `0.15`.

```lslx_reg\$plot_coefficient(block = "y<-y")
```

## Objects Extraction

In `lslx`, many quantities related to SEM can be extracted by extract-related method. For example, the coefficient under the penalty level selected by `aic` can be obtained by

```lslx_reg\$extract_coefficient(selector = "aic")
```

Here, `/g` means the coefficient belongs to the group `g` which is default group name. We may also check the quality of optimization by viewing the subgradient of objective function

```lslx_reg\$extract_objective_gradient(selector = "aic", type = "effective")
```

Here, the `type` argument is used to specify which types of parameters are used to calculate related quantities. `type = "effective"` indicates that only freely estimated and penalized non-zero parameters are used. By default, `type = "all"`. The subgradient shows that the obtained solution is optimal since all the elements are very small.

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lslx documentation built on April 28, 2020, 1:09 a.m.