# tikz in LaTeX and Structural Equation Modeling

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During grad school, I attended an ESA Workshop on Structural Equation Modeling (SEM) let by Jim Grace. The approach allows for multivariate analysis with multiple predictors, multiple response variables, and latent variables. Up until now, my research never required using the method and I never bought the software he recommended at the time because the GUI program recommended by Grace was too expensive for my limited needs.

Recently, I had a need to use SEM at work. We had two response variables: environmental DNA (eDNA) and the ash-free dry weight of an aquatic organism (AFDW). Both were predicted by multiple environmental variables and AFDW predicted eDNA. A perfect problem for SEM.

To refresh myself of SEM, I revisited Grace’s work. I discovered that he maintains an excellent tutorial about SEM. The pages provide a nice introduction, as does his (slightly outdated) book, his classic book, and a recent Ecoshephere article.

However, I did not have a nice way to plot my results. I did not want to use a WYSIWYG tool like Inkscape or Power Point. But I remembered the tikz package in LaTeX. Here’s the figure I created:

Example SEM plot.

I created the figure using this LaTeX code:

 \documentclass{article}

 \usepackage[paperheight =11.3cm, paperwidth =9.5cm, margin = 0.1cm]{geometry} \usepackage{tikz} \usetikzlibrary{arrows} \usetikzlibrary{positioning} \begin{document} \pagenumbering{gobble} \begin{tikzpicture}[ -> , >=stealth',auto,node distance=3.5cm, thick,main node/.style={rectangle,draw, font=\sffamily}] \node[main node] (1) {Lake}; \node[main node] (2) [below of=1] {Depth}; \node[main node] (3) [below of=2] {Non-habitat}; \node[main node] (4) [below of=3] {Habitat}; \node[main node] (6) [below right of=2, align = center] {AFDW\\ $$r^2 = 0.223$$}; \node[main node] (7) [right of=6, align = center] {eDNA\\ $$r^2 = 0.384$$}; \path[every node/.style={font=\sffamily\small}] (1) edge node [above = 40pt] {\textbf{0.497}} (6) (2) edge node [left = 10pt] {\textbf{-0.370}} (6) (3) edge node [above] {0.094} (6) (4) edge node [left = 10pt] {0.116} (6) (1) edge[bend left] node [above = 10 pt] {\textbf{0.385}} (7) (2) edge[bend left] node [above = 5pt ] {0.197} (7) (3) edge[bend right] node [above = 0pt] {-0.298} (7) (4) edge[bend right] node [below = 5pt] {0.204} (7) (6) edge node [ ] {-0.180} (7); \end{tikzpicture} 

\end{document} 

# 6 tips for a new LaTeX user

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Recently a coworker started using LaTeX and asked for some tips. Here’s my 6 tips for starting to use LaTeX:
1.  Start what you finish (i.e., close environments or else you get errors or weird bugs), for example $$needs an$$
2. Every document needs 3 things: \documentclass{<class>}, \begin{document}, and \end{document}
3. For equations, use  inline and \begin{eqnarray} \end{eqnarray} equations. \\ creates a new line. Use \\ \nonumber to continue on an equation and &=& to space multiple equations for example:
<code>
\begin{eqnarray}
a &=& b +c \\
a & =& b \\ \nonumber
& & c
\end{eqnarray}
</code>
Gives you something like:
a = b +c   (1)
a = b

c        (2)

4. Bib files are your friend for citations. Use Google Scholar to populate new citations.
5.  \textit{My italics text}, \textbf{my bold text}, should get most of your formatting. Do NOT use the depreciated {\bf bold} or {\it italics} style. (cf http://tex.stackexchange.com/questions/41681/correct-way-to-bold-italicize-text for more details on the second point)
6. {} can be very helpful, especially for complicated math functions, when order of operation is important. For example, \sigma_{\pi^2}^{2}