Drawing path diagrams with R package 'sem'

Question

I use the sem package to do my analysis. To draw a path diagram I use the pathDiagram function in sem to get the code and then use it in Graphviz to get the diagram. I have two questions:

library(sem)

R.DHP <- readMoments(diag=FALSE, names=c('ROccAsp', 'REdAsp', 'FOccAsp', 
                'FEdAsp', 'RParAsp', 'RIQ', 'RSES', 'FSES', 'FIQ', 'FParAsp'))
    .6247                                                              
    .3269  .3669                                                        
    .4216  .3275  .6404                                      
    .2137  .2742  .1124  .0839                                
    .4105  .4043  .2903  .2598  .1839                          
    .3240  .4047  .3054  .2786  .0489  .2220                    
    .2930  .2407  .4105  .3607  .0186  .1861  .2707              
    .2995  .2863  .5191  .5007  .0782  .3355  .2302  .2950        
    .0760  .0702  .2784  .1988  .1147  .1021  .0931 -.0438  .2087  

model.dhp <- specifyModel()
    RParAsp  -> RGenAsp, gam11,  NA
    RIQ      -> RGenAsp, gam12,  NA
    RSES     -> RGenAsp, gam13,  NA
    FSES     -> RGenAsp, gam14,  NA
    RSES     -> FGenAsp, gam23,  NA
    FSES     -> FGenAsp, gam24,  NA
    FIQ      -> FGenAsp, gam25,  NA
    FParAsp  -> FGenAsp, gam26,  NA
    FGenAsp  -> RGenAsp, beta12, NA
    RGenAsp  -> FGenAsp, beta21, NA
    RGenAsp  -> ROccAsp,  NA,       1
    RGenAsp  -> REdAsp,  lam21,  NA
    FGenAsp  -> FOccAsp,  NA,       1
    FGenAsp  -> FEdAsp,  lam42,  NA
    RGenAsp <-> RGenAsp, ps11,   NA
    FGenAsp <-> FGenAsp, ps22,   NA
    RGenAsp <-> FGenAsp, ps12,   NA
    ROccAsp <-> ROccAsp, theta1, NA
    REdAsp  <-> REdAsp,  theta2, NA
    FOccAsp <-> FOccAsp, theta3, NA
    FEdAsp  <-> FEdAsp,  theta4, NA



sem.dhp <- sem(model.dhp, R.DHP, 329,
    fixed.x=c('RParAsp', 'RIQ', 'RSES', 'FSES', 'FIQ', 'FParAsp'))



pathDiagram(sem.dhp, min.rank='RIQ, RSES, RParAsp, FParAsp, FSES, FIQ', 
    max.rank='ROccAsp, REdAsp, FEdAsp, FOccAsp')

1. How do I draw a path diagram directly from R using the pathDiagram function?
2. How do I include the graph in an Rnw file for Sweave?

Answer 1

You just need to specify a filename (without extension!), see the file= argument. As stated in the documentation, it will generated both a .dot and PDF file (but set output.type="dot" if you only want the graphviz output).

I would use a simple \includegraphics command in the Sweave file, after having called the above command. (You may need to adapt the path to find the figure if you don't generate the SEM diagram in the same directory as your master .Rnw file.)

Update

Given your comment, yes it seems there's a problem running external program from within the function call (pathDiagram). So here is a not very elegant solution to generate the path diagram and include it in your Sweave->TeX document.

Here is the Sweave file (sw.rnw):

\documentclass{article}
\usepackage{graphicx}
\begin{document}
<<echo=TRUE>>=
library(sem)
R.DHP <- readMoments("sem.cov", diag=FALSE, 
                     names=c('ROccAsp', 'REdAsp', 'FOccAsp', 
                       'FEdAsp', 'RParAsp', 'RIQ', 'RSES',  
                       'FSES', 'FIQ', 'FParAsp'))
model.dhp <- specifyModel(file="sem.mod")
sem.dhp <- sem(model.dhp, R.DHP, 329,
               fixed.x=c('RParAsp', 'RIQ', 'RSES', 'FSES', 'FIQ', 'FParAsp'))            
capture.output(pathDiagram(sem.dhp, min.rank='RIQ, RSES, RParAsp, FParAsp, FSES, FIQ', 
            max.rank='ROccAsp, REdAsp, FEdAsp, FOccAsp'), file="sem.dot")
@
<<echo=FALSE>>=
system("dot -Tpdf -o fig1.pdf  sem.dot")
@

And here is the path diagram.

\begin{center}
\includegraphics{fig1}
\end{center}

\end{document}

The files sem.cov and sem.mod contain the covariance matrix and structural model that were both entered manually in the above example (simple copy/paste in plain text file). I'm not very happy to have to use capture.output() because I cannot find a way to mask its call from the chunk. Maybe you'll find a better way to do that (the idea is to use system(), and that can easily be masked with echo=FALSE in the chunk parameters).

I happened to compile the above document as follows:

$ R CMD Sweave sw.rnw
$ R CMD texi2pdf sw.tex