The primary goal of GGMncv is to provide non-convex penalties for estimating Gaussian graphical models. These are known to overcome the various limitations of lasso (least absolute shrinkage “screening” operator), including inconsistent model selection (Zhao and Yu 2006), biased estimates (C.-H. Zhang 2010)[1], and a high false positive rate (see for example Williams and Rast 2020; Williams et al. 2019).

Note that these limitations of lasso are well-known. In the case of false positives, for example, it has been noted that

The lasso is doing variable screening and, hence, I suggest that we interpret the second ‘s’ in lasso as ‘screening’ rather than ‘selection.’ Once we have the screening property, the task is to remove the false positive selections (p. 278, Tibshirani 2011).

An additional goal of **GGMncv** is to provide methods for making statistical inference in **regularized** Gaussian graphical models. This is accomplished with the de-sparsified graphical lasso estimator introduced in Jankova and Van De Geer (2015). This is described in the section De-Sparsified Estimator. The next section shows how the de-sparsified estimator can be used to compare GGMs.

You can install the released version (`2.0.0`

) of **GGMncv** from CRAN with:

You can install development version from GitHub with:

The following are implemented in `GGMncv`

:

Atan (

`penalty = "atan"`

; Wang and Zhu (2016)). This is currently the default.Seamless

*L*_{0}(`penalty = "selo"`

; Dicker, Huang, and Lin (2013))Exponential (

`penalty = "exp"`

; Wang, Fan, and Zhu (2018))Smooth integration of counting and absolute deviation (

`penalty = "sica"`

; Lv and Fan (2009))Log (

`penalty = "log"`

; Mazumder, Friedman, and Hastie (2011))*L*_{q}(`penalty = "lq"`

,*0*<*q*<*1*; e.g., Knight and Fu (2000))Smoothly clipped absolute deviation (

`penalty = "scad"`

; Fan and Li (2001))Minimax concave penalty (

`penalty = "mcp"`

; C.-H. Zhang (2010))Adaptive lasso (

`penalty = "adapt"`

; Zou (2006))Lasso (

`penalty = "lasso"`

; Tibshirani (1996))

Options 1-5 are continuous approximations to the *L*_{0} penalty, that is, best subsets model selection. However, the solution is computationally efficient and solved with the local linear approximation described in Fan, Feng, and Wu (2009) or the one-step approach described in Zou and Li (2008).

The basic idea of these penalties is to provide “tapering,” in which regularization is less severe for large effects. The following is an example for the Atan penalty (

Note that (1) the penalty provides a “smooth” function that ranges from *L*_{0} (best subsets) and *L*_{1} (lasso) regularization; and (2) the penalty “tapers” off for large effects.

Computing the non-convex solution is a challenging task. However, section 3.3 in Zou and Li (2008) indicates that the one-step approach is a viable **approximation** for a variety of non-convex penalties, assuming the initial estimates are “good enough”[2]. To this end, the initial values can either be the sample based inverse covariance matrix or a custom matrix specified with `initial`

.

The tuning parameter can be selected with several information criteria (IC), including `aic`

, `bic`

(currently the default),`ebic`

, `ric`

, in addition to any of the *generalized* information criteria provided in section 5 of Kim, Kwon, and Choi (2012).

Information criterion can be understood as penalizing the likelihood, with the difference being in the severity of the penalty. -2 times the log-likelihood is defined as

where

- GIC
_{1}(BIC):

Note that

- GIC
_{2}:

*p* denotes the number of nodes or columns in the data matrix.

GIC

_{3}(RIC):GIC

_{4}:GIC

_{5}(BIC with divergent dimensions):GIC

_{6}:AIC:

Although cross-validation is not implemented for selecting the tuning parameter, AIC can be used to approximate leave-one-out cross-validation.

- EBIC:

The tuning parameter is selected by setting `select = TRUE`

and then the desired IC with, for example, `ic = "gic_3"`

.

A tuning free option is also available. This is accomplished by setting the tuning parameter to

A GGM can be fitted as follows

```
library(GGMncv)
# data
Y <- GGMncv::ptsd[,1:10]
# polychoric
R <- cor(Y, method = "spearman")
# fit model
fit <- ggmncv(R = R, n = nrow(Y),
penalty = "atan")
# print
fit
#> 1 2 3 4 5 6 7 8 9 10
#> 1 0.000 0.255 0.000 0.309 0.101 0.000 0.000 0.000 0.073 0.000
#> 2 0.255 0.000 0.485 0.000 0.000 0.000 0.122 0.000 0.000 0.000
#> 3 0.000 0.485 0.000 0.185 0.232 0.000 0.000 0.000 0.000 0.000
#> 4 0.309 0.000 0.185 0.000 0.300 0.000 0.097 0.000 0.000 0.243
#> 5 0.101 0.000 0.232 0.300 0.000 0.211 0.166 0.000 0.000 0.000
#> 6 0.000 0.000 0.000 0.000 0.211 0.000 0.234 0.079 0.000 0.000
#> 7 0.000 0.122 0.000 0.097 0.166 0.234 0.000 0.000 0.000 0.000
#> 8 0.000 0.000 0.000 0.000 0.000 0.079 0.000 0.000 0.000 0.114
#> 9 0.073 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.261
#> 10 0.000 0.000 0.000 0.243 0.000 0.000 0.000 0.114 0.261 0.000
```

Note that the object `fit`

can be plotted with the `R`

package **qgraph**.

The **GGMncv** package can also be used for prediction, given the correspondence between the inverse covariance matrix and multiple regression (Kwan 2014).

```
Y <- scale(Sachs)
# test data
Ytest <- Y[1:100,]
# training data
Ytrain <- Y[101:nrow(Y),]
# default: atan and tuning free
fit <- ggmncv(cor(Ytrain),
n = nrow(Ytrain))
# predict
pred <- predict(fit, train_data = Ytrain, newdata = Ytest)
# print mse
round(apply((pred - Ytest)^2, 2, mean), 2)
#> Raf Erk Plcg PKC PKA PIP2 PIP3 Mek P38 Jnk Akt
#> 0.18 0.27 0.59 0.42 0.39 0.47 0.69 0.16 0.15 0.69 0.26
```

When `select = 'lambda'`

, the solution path for the partial correlations can be plotted.

Here is the current default penalty

```
# data
Y <- ptsd
# fit model
fit <- GGMncv(cor(Y), n = nrow(Y),
select = TRUE,
store = TRUE)
# plot path
plot(fit,
alpha = 0.75)
```

The dotted line is denotes the selected lambda. Notice how the larger partial correlations “escape” regularization, at least to some degree, compared to the smaller partial correlations.

Next *L*_{1} regularization is implemented by setting `penalty = "lasso"`

.

```
# data
Y <- ptsd
# fit model
fit <- GGMncv(cor(Y), n = nrow(Y),
penalty = "lasso"
store = TRUE)
# plot path
plot(fit,
alpha = 0.75)
```

This solution is much different than above. For example, it is clear that the large partial correlations are heavily penalized, whereas this was not so for the atan penalty. The reason this is not ideal is that, if the partial correlations are large, it makes sense that they should not be penalized that much. This property of non-convex regularization should provide *nearly* unbiased estimates, which can improve, say, predictive accuracy.

Also notice that the atan penalty provides a sparser solution.

**GGMncv** does not provide confidence intervals based on bootstrapping.

This is because, in general, “confidence” intervals from penalized approaches do not have the correct properties to be considered confidence intervals (see Wikipedia). This sentiment is echoed in Section 3.1, “Why standard bootstrapping and subsampling do not work,” of Bühlmann, Kalisch, and Meier (2014):

The (limiting) distribution of such a sparse estimator is non-Gaussian with point mass at zero, and this is the reason why standard bootstrap or subsampling techniques do not provide valid confidence regions or p-values (pp. 7-8).

For this reason, it is common to **not** provide standard errors (and thus confidence intervals) for penalized models [3]. For example, this is from the **penalized** `R`

package:

It is a very natural question to ask for standard errors of regression coefficients or other estimated quantities. In principle such standard errors can easily be calculated, e.g. using the bootstrap. Still, this package deliberately does not provide them. The reason for this is that standard errors are not very meaningful for strongly biased estimates such as arise from penalized estimation methods (p.18, Goeman, Meijer, and Chaturvedi 2018)

However, **GGMncv** does include the so-called variable inclusion “probability” for each relation (see p. 1523 in Bunea et al. 2011; and Figure 6.7 in Hastie, Tibshirani, and Wainwright 2015). These are computed using a non-parametric bootstrap strategy.

Additionally, more recent work does allow for obtaining confidence intervals and *p*-values with the de-sparsified method. For the graphical lasso, the former are not available for the partial correlations so currently only *p*-values are provided (Statistical Inferece).

It might be tempting to think these approaches lead to rich *inference*. This would be a mistake–they suffer from all of the problems inherent to automated procedures for model selection (e.g., Berk et al. 2013; Lee et al. 2016).

And note that:

Simply

**not**detecting an effect does not provide evidence for the null hypothesis.There is

**not**necessarily a difference between an effect that was and an effect that was not detected.

Supporting these claims would require a valid confidence interval that has been corrected for model selection and/or regularization. With these caveats in mind, data driven model selection in **GGMncv** can be used for explicit data mining or prediction.

To make inference, **GGMncv** computes the de-sparsified estimator,

where

which readily allows for computing *p*-values for each off-diagonal element of the de-sparsified estimator.

This is implemented with

```
# data
Y <- ptsd
# fit model
fit <- ggmncv(cor(Y), n = nrow(Y))
# make inference
fdr_ggm <- inference(fit, method = "fdr")
# print
fdr_ggm
#> Statistical Inference
#> fdr: 0.05
#> ---
#> 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
#> 1 0 1 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0
#> 2 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0
#> 3 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> 4 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> 5 0 0 0 1 0 1 0 0 0 0 0 0 0 0 1 0 0 1 0 0
#> 6 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0
#> 7 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0
#> 8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
#> 9 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0
#> 10 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0
#> 11 1 0 0 0 0 0 0 0 1 1 0 0 0 0 1 0 0 0 0 0
#> 12 0 0 0 0 0 0 1 0 1 0 0 0 1 0 0 0 0 0 1 0
#> 13 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 1 0
#> 14 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0
#> 15 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0
#> 16 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0
#> 17 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0
#> 18 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0
#> 19 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1
#> 20 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0
```

Note that the object `fdr_ggm`

includes the de-sparsified precision matrix, the partial correlation matrix, and *p*-values for each relation. Furthermore, there is a function called `desparsify()`

that can be used to obtain the de-sparsified estimator without computing the *p*-values.

Because the de-sparsified estimator provides the variance for each relation, this readily allows for comparing GGMs. This is accomplished by computing the difference and then the variance of that difference. Assuming there is two groups, `Y_g1`

and `Y_g2`

, this is implemented with

```
fit1 <- ggmncv(Y_g1, n = nrow(Y_g1))
fit2 <- ggmncv(Y_g2, n = nrow(Y_g2))
ggm_diff <- ggm_compare(fit1, fit2)
```

The object `ggm_diff`

includes the partial correlation differences, *p*-values, and the adjacency matrix.

It is important to note that **GGMncv** merely provides a software implementation of other researchers work. There are no methological innovations, although this is the most comprehensive `R`

package for estimating GGMs with non-convex penalties. Hence, in addition to citing the package `citation("GGMncv")`

, it is important to give credit to the primary sources. The references can be found in (Penalties).

Additionally, please cite Williams (2020) which is survey of these approaches that is meant to accompany **GGMncv**.

Note that the penalties in

**GGMncv**should provide*nearly*unbiased estimates (return).In low-dimensional settings, assuming that

*n*is sufficiently larger than*p*, the sample covariance matrix provides adequate initial estimates. In high-dimensional settings (*n*<*p*), the initial estimates are obtained from lasso (return).It is possible to compute confidence intervals for lasso with the methods included in the

**SILGGM**`R`

package (R. Zhang, Ren, and Chen 2018). These do not use the bootstrap (return).

Berk, Richard, Lawrence Brown, Andreas Buja, Kai Zhang, Linda Zhao, et al. 2013. “Valid Post-Selection Inference.” *The Annals of Statistics* 41 (2): 802–37.

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Bunea, Florentina, Yiyuan She, Hernando Ombao, Assawin Gongvatana, Kate Devlin, and Ronald Cohen. 2011. “Penalized Least Squares Regression Methods and Applications to Neuroimaging.” *NeuroImage* 55 (4): 1519–27.

Dicker, Lee, Baosheng Huang, and Xihong Lin. 2013. “Variable Selection and Estimation with the Seamless-l 0 Penalty.” *Statistica Sinica*, 929–62.

Fan, Jianqing, Yang Feng, and Yichao Wu. 2009. “Network Exploration via the Adaptive LASSO and SCAD Penalties.” *The Annals of Applied Statistics* 3 (2): 521.

Fan, Jianqing, and Runze Li. 2001. “Variable Selection via Nonconcave Penalized Likelihood and Its Oracle Properties.” *Journal of the American Statistical Association* 96 (456): 1348–60.

Goeman, Jelle, Rosa Meijer, and Nimisha Chaturvedi. 2018. “L1 and L2 Penalized Regression Models.” *Vignette R Package Penalized.*

Hastie, Trevor, Robert Tibshirani, and Martin Wainwright. 2015. *Statistical Learning with Sparsity: The Lasso and Generalizations*. CRC press.

Jankova, Jana, and Sara Van De Geer. 2015. “Confidence Intervals for High-Dimensional Inverse Covariance Estimation.” *Electronic Journal of Statistics* 9 (1): 1205–29.

Kim, Yongdai, Sunghoon Kwon, and Hosik Choi. 2012. “Consistent Model Selection Criteria on High Dimensions.” *The Journal of Machine Learning Research* 13: 1037–57.

Knight, Keith, and Wenjiang Fu. 2000. “Asymptotics for Lasso-Type Estimators.” *Annals of Statistics*, 1356–78.

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