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• Covariance of non-normal data
  • Continuous but not normal
  • Ordinal
• Computing Markov Random Fields
  • Recap
  • Toy example
  • Estimating sparse models with the LASSO

• The Gaussian Random Field assumes normally distributed data
  • What if this is not the case?
• Two types of data that might not be normal:
  • Continuous data that is not normally distributed
  • Ordinal data with relatively few possible outcomes
    • Likert scales

• If data is continuous but not normal, the nonparanormal transformation can be used to transform data to be normal
  • Semi-parametric transformation
  • huge.npn function in the huge package

DF <- data.frame(
  x = ifelse(sample(c(TRUE,FALSE), 1000, TRUE), 
             rnorm(1000,-10,4), rexp(1000,20)),
  y = rbeta(1000, 0.5, 0.5)
)

library('ggplot2')
ggplot(DF, aes(x = x)) + geom_density()

ggplot(DF, aes(x = y)) + geom_density()

ggplot(DF, aes(x = x, y = y )) + geom_point()

Transform to normal data:

library('huge')
DFnormal <- as.data.frame(huge.npn(DF))

## Conducting the nonparanormal (npn) transformation via shrunkun ECDF....done.

library('ggplot2')
ggplot(DFnormal, aes(x = x)) + geom_density()

ggplot(DFnormal, aes(x = y)) + geom_density()

ggplot(DFnormal, aes(x = x, y = y )) + geom_point()

• If data is ordinal and consists of only a few levels of measurement the nonparanormal transformation does not work
• In this case threshold models should be used
• Then, it is assumed that underlying the response is a latent item that is normally distributed
• The covariance between this latent items and other such latent items or other continuous items can be estimated
  • Polychoric correlation if both variables are ordinal
  • Polyserial correlation if one item is ordinal and the other is continuous

I see myself as someone who is talkative…

I see myself as someone who is talkative…

• The cor_auto() function in the qgraph package can be used to compute a correlation matrix of variables of which some are ordinal
  • By default searchers for variables with integer values of at most \(7\) different levels
• Computes polychoric, polyserial and Pearson correlations
• Can be used in replacement of cor!

# True correlation matrix:
Cor <- matrix(c(1,0.5,0.25,
                0.5,1,0.5,
                0.25,0.5,1),3,3)

Cor

##      [,1] [,2] [,3]
## [1,] 1.00  0.5 0.25
## [2,] 0.50  1.0 0.50
## [3,] 0.25  0.5 1.00

# Simulate true scores:
library(mvtnorm)

## Warning: package 'mvtnorm' was built under R version 3.1.1

Ylat <- rmvnorm(1000,c(0,0,0),Cor)

# Discretize variables 1 and 2
Y <- Ylat
Y[,1:2] <- as.numeric(cut(Ylat[,1:2], breaks = c(-Inf,0,1,Inf)))

head(Y,10)

##       [,1] [,2]     [,3]
##  [1,]    1    1  0.02917
##  [2,]    2    2  1.73107
##  [3,]    1    1 -1.58392
##  [4,]    1    1 -0.01996
##  [5,]    1    2  0.61200
##  [6,]    2    3 -0.30646
##  [7,]    3    3  0.51575
##  [8,]    3    1 -1.46357
##  [9,]    2    3  0.79041
## [10,]    2    1  1.27870

# True correlation matrix:
Cor

##      [,1] [,2] [,3]
## [1,] 1.00  0.5 0.25
## [2,] 0.50  1.0 0.50
## [3,] 0.25  0.5 1.00

# Pearson correlations:
cor(Y)

##        [,1]   [,2]   [,3]
## [1,] 1.0000 0.3563 0.2276
## [2,] 0.3563 1.0000 0.4682
## [3,] 0.2276 0.4682 1.0000

• Pearson correlations underestimate the true correlation!

# True correlation matrix:
Cor

##      [,1] [,2] [,3]
## [1,] 1.00  0.5 0.25
## [2,] 0.50  1.0 0.50
## [3,] 0.25  0.5 1.00

# Polychoric correlations
cor_auto(Y)

## Variables detected as ordinal: V1; V2

##        V1     V2     V3
## V1 1.0000 0.4499 0.2576
## V2 0.4499 1.0000 0.5393
## V3 0.2576 0.5393 1.0000

• Polychoric correlations work much better!

• Ising Model
  • All nodes are binary
    • \(-1\) or \(1\)
  • Combination of multiple logistic regression models
• Gaussian Random Field
  • All nodes assumed normally distributed
  • Graph structure directly related to the inverse variance-covariance matrix
  • Graph usually standardized to partial correlations
  • Combination of multiple linear regression models

• Because the Ising Model is a combination of multiple logistic models and the Gaussian Random field a combination of multiple linear models we can form MRF's in the following way:

  1. Predict each node from all other nodes in a multiple (logistic) regression
  2. This gives you two parameters for each edge in the network. For example:
     • The regression parameter from predicting \(A\) from \(B\) and the regression parameter from \(B\) to \(A\).
     • In logistic regression, these should be about equal. In linear regression, they can be standardized to be about equal
  3. Take the mean of these two parameters as edge weight
     • If the regression parameter is \(0\) there is no edge!

\[ \Pr(X_1 = 1) \propto \exp\left( \tau_1 + \beta_{12} x_2 + \beta_{13} x_3 + \beta_{14} x_4 \right) \]

\[ \Pr(X_2 = 1) \propto \exp\left( \tau_2 + \beta_{21} x_1 + \beta_{23} x_3 + \beta_{24} x_4 \right) \]

\[ \Pr(X_3 = 1) \propto \exp\left( \tau_3 + \beta_{31} x_1 + \beta_{32} x_2 + \beta_{34} x_4 \right) \]

\[ \Pr(X_4 = 1) \propto \exp\left( \tau_4 + \beta_{41} x_1 + \beta_{42} x_2 + \beta_{43} x_3 \right) \]

\[ \omega_{ij} = \beta_{ij} = \beta_{ji} \]

\[ X_1 = \tau_1 + \beta_{12} x_2 + \beta_{13} x_3 + \beta_{14} x_4 + \varepsilon_1 \]

\[ X_2 = \tau_2 + \beta_{21} x_1 + \beta_{23} x_3 + \beta_{24} x_4 + \varepsilon_2 \]

\[ X_3 = \tau_3 + \beta_{31} x_1 + \beta_{32} x_2 + \beta_{34} x_4 + \varepsilon_3 \]

\[ X_4 = \tau_4 + \beta_{41} x_1 + \beta_{42} x_2 + \beta_{43} x_3 + \varepsilon_4 \]

\[ \rho_{ij} = \frac{\beta_{ij} \mathrm{Var}\left(\varepsilon_j\right)}{\mathrm{Var}\left(\varepsilon_i\right)} = \frac{\beta_{ji} \mathrm{Var}\left(\varepsilon_i\right)}{\mathrm{Var}\left(\varepsilon_j\right)} \]

\[ \rho_{ij} = -\frac{ \theta_{ij} }{\sqrt{ \theta_{ii} \theta_{jj} }} \hspace{35pt} \boldsymbol{\Theta} = \boldsymbol{\Sigma}^{-1} \]

In which \(\boldsymbol{\Theta}\) is the precision matrix; the inverse of the variance-covariance matrix.

• In these slides I will show how to estimate a Gaussian Random Field on a simulated toy dataset:
  • \(W\): Worrying
  • \(C\): Concentration problems
  • \(F\): Fatigue
  • \(I\): Insomnia
• The same logic applies to the Ising Model

Data <- structure(list(W = c(4, 1, 5, 4, 2, 2, 8, 1, 4, 4), C = c(2, 
6, 4, 2, 9, 6, 2, 5, 4, 2), F = c(3, 2, 4, 8, 1, 2, 4, 4, 5, 
5), I = c(1, 1, 6, 8, 3, 5, 8, 1, 3, 4)), .Names = c("W", "C", 
"F", "I"), row.names = c(NA, -10L), class = "data.frame")

Data

##    W C F I
## 1  4 2 3 1
## 2  1 6 2 1
## 3  5 4 4 6
## 4  4 2 8 8
## 5  2 9 1 3
## 6  2 6 2 5
## 7  8 2 4 8
## 8  1 5 4 1
## 9  4 4 5 3
## 10 4 2 5 4

round(cor(Data),2)

##       W     C     F     I
## W  1.00 -0.67  0.40  0.70
## C -0.67  1.00 -0.73 -0.38
## F  0.40 -0.73  1.00  0.52
## I  0.70 -0.38  0.52  1.00

library("qgraph")
qgraph(cor(Data), esize = 20)

• We can construct a concentration network by predicting all variables from all other variables:
  • \(W = \beta_{12} C + \beta_{13} F + \beta_{14} I + \varepsilon_W\)
  • \(C = \beta_{21} W + \beta_{23} F + \beta_{24} I + \varepsilon_C\)
  • \(F = \beta_{31} W + \beta_{32} C + \beta_{34} I + \varepsilon_F\)
  • \(I = \beta_{41} W + \beta_{42} C + \beta_{43} F + \varepsilon_I\)
• In which for all errors \(\varepsilon_i \sim N(0, \theta_i)\) and are assumed to be independent of all other nodes.

fitW <- lm(W ~ C + F + I, data = Data)
fitW

## 
## Call:
## lm(formula = W ~ C + F + I, data = Data)
## 
## Coefficients:
## (Intercept)            C            F            I  
##       6.606       -0.723       -0.564        0.518

fitC <- lm(C ~ W + F + I, data = Data)
fitF <- lm(F ~ W + C + I, data = Data)
fitI <- lm(I ~ W + C + F, data = Data)

\[ \begin{aligned} W &= \beta_{12} C + \beta_{13} F + \beta_{14} I + \varepsilon_W \\ C &= \beta_{21} W + \beta_{23} F + \beta_{24} I + \varepsilon_C \\ F &= \beta_{31} W + \beta_{32} C + \beta_{34} I + \varepsilon_F \\ I &= \beta_{41} W + \beta_{42} C + \beta_{43} F + \varepsilon_I \end{aligned} \]

• The regression coefficients have a symmetrical relationship:
  • \(\beta_{ij} \mathrm{Var}\left( \varepsilon_j \right) = \beta_{ji} \mathrm{Var}\left( \varepsilon_i \right)\)

beta_WC <- coef(fitW)[2]
beta_CW <- coef(fitC)[2]
errorVariance_W <- summary(fitW)$sigma^2
errorVariance_C <- summary(fitC)$sigma^2

beta_WC * errorVariance_C

##      C 
## -1.163

beta_CW * errorVariance_W

##      W 
## -1.163

\[ \begin{aligned} W = {\color{red} \beta_{12}} C + {\color{blue} \beta_{13}} F + {\color{green}\beta_{14}} I + \varepsilon_W \\ C = {\color{red}\beta_{21}} W + {\color{purple} \beta_{23} } F + {\color{orange} \beta_{24}} I + \varepsilon_C \\ F = {\color{blue} \beta_{31}} W + {\color{purple} \beta_{32} } C + {\color{brown}\beta_{34}} I + \varepsilon_F \\ I = {\color{green}\beta_{41}} W + {\color{orange} \beta_{42} } C + {\color{brown} \beta_{43}} F + \varepsilon_I \\ \end{aligned} \]

• These regression parameters can be standardized to obtain the partial correlation coefficient:
  • \(\rho_{ij} = \frac{\beta_{ij} \sqrt{ \mathrm{Var}\left( \varepsilon_j \right)}}{\sqrt{ \mathrm{Var}\left( \varepsilon_i \right)}} = \frac{\beta_{ji} \sqrt{ \mathrm{Var}\left( \varepsilon_i \right)}}{\sqrt{ \mathrm{Var}\left( \varepsilon_j \right)}}\)
• Are equal to standardized elements of the negative inverse covariance matrix!
  • \(\rho_{ij} = -\frac{ \theta_{ij} }{\sqrt{ \theta_{ii} \theta_{jj} }} \hspace{35pt} \boldsymbol{\Theta} = \boldsymbol{\Sigma}^{-1}\)

beta_WC * sqrt(errorVariance_C) / sqrt(errorVariance_W)

##       C 
## -0.7651

beta_CW * sqrt(errorVariance_W) / sqrt(errorVariance_C)

##       W 
## -0.7651

-cov2cor(solve(cov(Data)))

##         W       C       F       I
## W -1.0000 -0.7651 -0.5889  0.7754
## C -0.7651 -1.0000 -0.7993  0.5871
## F -0.5889 -0.7993 -1.0000  0.6468
## I  0.7754  0.5871  0.6468 -1.0000

These partial correlations give us the concentration graph:

library("qgraph")
qgraph(cor(Data), graph = "concentration")

• Why does this still not look good?

Data

##    W C F I
## 1  4 2 3 1
## 2  1 6 2 1
## 3  5 4 4 6
## 4  4 2 8 8
## 5  2 9 1 3
## 6  2 6 2 5
## 7  8 2 4 8
## 8  1 5 4 1
## 9  4 4 5 3
## 10 4 2 5 4