**Data, Evidence, and Policy - Jared Knowles**, and kindly contributed to R-bloggers)

# Getting Started with Structural Equation Modeling: Part 1

# Introduction

For the analyst familiar with linear regression fitting structural equation models can at first feel strange. In the R environment, fitting structural equation models involves learning new modeling syntax, new plotting syntax, and often a new data input method. However, a quick reorientation and soon the user is exposed to the differences, fitting structural equation models can be a powerful tool in the analyst's toolkit.

This tutorial will cover getting set up and running a few basic models using `lavaan`

in R.1 Future tutorials will cover:

- constructing latent variables
- comparing alternate models
- multi-group analysis on larger datasets.

# Setting up your enviRonment

Getting started using structural equation modeling (SEM) in R can be daunting. There are lots of different packages for implementing SEM in R and there are different features of SEM that a user might be interested in implementing. A few packages you might come across can be found on the CRAN Psychometrics Task View.

For those who want to just dive in the `lavaan`

package seems to offer the most
comprehensive feature set for most SEM users and has a well thought out and easy
to learn syntax for describing SEM models. To install `lavaan`

, we just run:

```
# Main version
install.packages("lavaan")
# Or to install the dev version
library(devtools)
install_github("lavaan", "yrosseel")
```

# Read in the data

Once we load up the lavaan package, we need to read in the dataset. `lavaan`

accepts
two different types of data, either a standard R dataframe, or a variance-covariance
matrix. Since the latter is unfamiliar to us coming from the standard `lm`

linear modeling
framework in R, we'll start with reading in the simplest variance-covariance matrix
possible and running a path analysis model.

```
library(lavaan)
mat1 <- matrix(c(1, 0, 0, 0.6, 1, 0, 0.33, 0.63, 1), 3, 3, byrow = TRUE)
colnames(mat1) <- rownames(mat1) <- c("ILL", "IMM", "DEP")
myN <- 500
print(mat1)
```

```
## ILL IMM DEP
## ILL 1.00 0.00 0
## IMM 0.60 1.00 0
## DEP 0.33 0.63 1
```

```
# Note that we only input the lower triangle of the matrix. This is
# sufficient though we could put the whole matrix in if we like
```

Now we have a variance-covariance matrix in our environment named `mat1`

and a
variable `myN`

corresponding to the number of observations in our dataset. Alternatively,
we could provide R with the full dataset and it can derive `mat1`

and `myN`

itself.

With this data we can construct two possible models:

- Depression (DEP) influences Immune System (IMM) influences Illness (ILL)
- IMM influences ILL influences DEP

Using SEM we can evaluate which model best explains the covariances we observe in
our data above. Fitting models in `lavaan`

is a two step process. First, we create
a text string that serves as the `lavaan`

model and follows the `lavaan`

model
syntax. Next, we
give `lavaan`

the instructions on how to fit this model to the data using either the `cfa`

, `lavaan`

, or `sem`

functions. Here we will use the `sem`

function. Other
functions will be covered in a future post.

```
# Specify the model
mod1 <- "ILL ~ IMM \n IMM ~ DEP"
# Give lavaan the command to fit the model
mod1fit <- sem(mod1, sample.cov = mat1, sample.nobs = 500)
# Specify model 2
mod2 <- "DEP ~ ILL\n ILL ~ IMM"
mod2fit <- sem(mod2, sample.cov = mat1, sample.nobs = 500)
```

Now we have two objects stored in our environment for each model. We have the
model string and the modelfit object. The model fit objects (`mod1fit`

and `mod2fit`

)
are `lavaan`

class objects. These are S4 objects with many supported methods, including
the `summary`

method which provides a lot of useful output:

```
# Summarize the model fit
summary(mod1fit)
```

```
## lavaan (0.5-14) converged normally after 12 iterations
##
## Number of observations 500
##
## Estimator ML
## Minimum Function Test Statistic 2.994
## Degrees of freedom 1
## P-value (Chi-square) 0.084
##
## Parameter estimates:
##
## Information Expected
## Standard Errors Standard
##
## Estimate Std.err Z-value P(>|z|)
## Regressions:
## ILL ~
## IMM 0.600 0.036 16.771 0.000
## IMM ~
## DEP 0.630 0.035 18.140 0.000
##
## Variances:
## ILL 0.639 0.040
## IMM 0.602 0.038
```

```
summary(mod2fit)
```

```
## lavaan (0.5-14) converged normally after 11 iterations
##
## Number of observations 500
##
## Estimator ML
## Minimum Function Test Statistic 198.180
## Degrees of freedom 1
## P-value (Chi-square) 0.000
##
## Parameter estimates:
##
## Information Expected
## Standard Errors Standard
##
## Estimate Std.err Z-value P(>|z|)
## Regressions:
## DEP ~
## ILL 0.330 0.042 7.817 0.000
## ILL ~
## IMM 0.600 0.036 16.771 0.000
##
## Variances:
## DEP 0.889 0.056
## ILL 0.639 0.040
```

One of the best ways to understand an SEM model is to inspect the model visually
using a path diagram. Thanks to the `semPlot`

package,
this is easy to do in R.2 First, install `semPlot`

:

```
# Official version
install.packages("semPlot")
# Or to install the dev version
library(devtools)
install_github("semPlot", "SachaEpskamp")
```

Next we load the library and make some path diagrams.

```
library(semPlot)
semPaths(mod1fit, what = "est", layout = "tree", title = TRUE, style = "LISREL")
```

```
semPaths(mod2fit, what = "est", layout = "tree", title = TRUE, style = "LISREL")
```

These two simple path models look great. But which is better? We can run a simple
chi-square test on the `lavaan`

objects `mod1fit`

and `mod2fit`

.

```
anova(mod1fit, mod2fit)
```

```
## Chi Square Difference Test
##
## Df AIC BIC Chisq Chisq diff Df diff Pr(>Chisq)
## mod1fit 1 3786 3803 2.99
## mod2fit 1 3981 3998 198.18 195 0 <2e-16 ***
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
```

We can see that very clearly we prefer Model 2. Let's look at some properties of
model 2 that we can access through the `lavaan`

object with convenience functions.

```
# Goodness of fit measures
fitMeasures(mod2fit)
```

```
## fmin chisq df pvalue
## 0.198 198.180 1.000 0.000
## baseline.chisq baseline.df baseline.pvalue cfi
## 478.973 3.000 0.000 0.586
## tli nnfi rfi nfi
## -0.243 -0.243 1.000 0.586
## pnfi ifi rni logl
## 0.195 0.587 0.586 -1986.510
## unrestricted.logl npar aic bic
## -1887.420 4.000 3981.020 3997.878
## ntotal bic2 rmsea rmsea.ci.lower
## 500.000 3985.182 0.628 0.556
## rmsea.ci.upper rmsea.pvalue rmr rmr_nomean
## 0.703 0.000 0.176 0.176
## srmr srmr_nomean cn_05 cn_01
## 0.176 0.176 10.692 17.740
## gfi agfi pgfi mfi
## 0.821 -0.075 0.137 0.821
## ecvi
## 0.412
```

```
# Estimates of the model parameters
parameterEstimates(mod2fit, ci = TRUE, boot.ci.type = "norm")
```

```
## lhs op rhs est se z pvalue ci.lower ci.upper
## 1 DEP ~ ILL 0.330 0.042 7.817 0 0.247 0.413
## 2 ILL ~ IMM 0.600 0.036 16.771 0 0.530 0.670
## 3 DEP ~~ DEP 0.889 0.056 15.811 0 0.779 1.000
## 4 ILL ~~ ILL 0.639 0.040 15.811 0 0.560 0.718
## 5 IMM ~~ IMM 0.998 0.000 NA NA 0.998 0.998
```

```
# Modification indices
modindices(mod2fit, standardized = TRUE)
```

```
## lhs op rhs mi epc sepc.lv sepc.all sepc.nox
## 1 DEP ~~ DEP 0.0 0.000 0.000 0.000 0.000
## 2 DEP ~~ ILL 163.6 -0.719 -0.719 -0.720 -0.720
## 3 DEP ~~ IMM 163.6 0.674 0.674 0.675 0.674
## 4 ILL ~~ ILL 0.0 0.000 0.000 0.000 0.000
## 5 ILL ~~ IMM NA NA NA NA NA
## 6 IMM ~~ IMM 0.0 0.000 0.000 0.000 0.000
## 7 DEP ~ ILL 0.0 0.000 0.000 0.000 0.000
## 8 DEP ~ IMM 163.6 0.675 0.675 0.675 0.676
## 9 ILL ~ DEP 163.6 -0.808 -0.808 -0.808 -0.808
## 10 ILL ~ IMM 0.0 0.000 0.000 0.000 0.000
## 11 IMM ~ DEP 143.8 0.666 0.666 0.666 0.666
## 12 IMM ~ ILL 0.0 0.000 0.000 0.000 0.000
```

That's it. From inputing a variance-covariance matrix to fitting a model,
drawing a path diagram, comparing to alternate models, and finally inspecting
the parameters of the preferred model. `lavaan`

is an amazing project which
adds great capabilities to R. These will be explored in future posts.

# Appendix

```
citation(package = "lavaan")
```

```
##
## To cite lavaan in publications use:
##
## Yves Rosseel (2012). lavaan: An R Package for Structural
## Equation Modeling. Journal of Statistical Software, 48(2), 1-36.
## URL http://www.jstatsoft.org/v48/i02/.
##
## A BibTeX entry for LaTeX users is
##
## @Article{,
## title = {{lavaan}: An {R} Package for Structural Equation Modeling},
## author = {Yves Rosseel},
## journal = {Journal of Statistical Software},
## year = {2012},
## volume = {48},
## number = {2},
## pages = {1--36},
## url = {http://www.jstatsoft.org/v48/i02/},
## }
```

```
citation(package = "semPlot")
```

```
##
## To cite package 'semPlot' in publications use:
##
## Sacha Epskamp (2013). semPlot: Path diagrams and visual analysis
## of various SEM packages' output. R package version 0.3.3.
## https://github.com/SachaEpskamp/semPlot
##
## A BibTeX entry for LaTeX users is
##
## @Manual{,
## title = {semPlot: Path diagrams and visual analysis of various SEM packages' output},
## author = {Sacha Epskamp},
## year = {2013},
## note = {R package version 0.3.3},
## url = {https://github.com/SachaEpskamp/semPlot},
## }
##
## ATTENTION: This citation information has been auto-generated from
## the package DESCRIPTION file and may need manual editing, see
## 'help("citation")'.
```

```
sessionInfo()
```

```
## R version 3.0.1 (2013-05-16)
## Platform: x86_64-w64-mingw32/x64 (64-bit)
##
## locale:
## [1] LC_COLLATE=English_United States.1252
## [2] LC_CTYPE=English_United States.1252
## [3] LC_MONETARY=English_United States.1252
## [4] LC_NUMERIC=C
## [5] LC_TIME=English_United States.1252
##
## attached base packages:
## [1] stats graphics grDevices utils datasets methods base
##
## other attached packages:
## [1] semPlot_0.3.3 lavaan_0.5-14 quadprog_1.5-5 pbivnorm_0.5-1
## [5] mnormt_1.4-5 boot_1.3-9 MASS_7.3-28 knitr_1.4.1
##
## loaded via a namespace (and not attached):
## [1] car_2.0-18 cluster_1.14.4 colorspace_1.2-2 corpcor_1.6.6
## [5] digest_0.6.3 ellipse_0.3-8 evaluate_0.4.7 formatR_0.9
## [9] grid_3.0.1 Hmisc_3.12-2 igraph_0.6.5-2 jpeg_0.1-6
## [13] lattice_0.20-23 lisrelToR_0.1.4 plyr_1.8 png_0.1-6
## [17] psych_1.3.2 qgraph_1.2.3 rockchalk_1.8.0 rpart_4.1-2
## [21] sem_3.1-3 stats4_3.0.1 stringr_0.6.2 tools_3.0.1
## [25] XML_3.98-1.1
```

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**Data, Evidence, and Policy - Jared Knowles**.

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