FYI: Info for An Introduction to SEM for Ecologists
Published by 劉正山,
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Many problems in ecology and evolutionary biology require understanding of the relationships among variables and examining their relative influences and responses. For example, over the last few decades ecologists have been trying to quantify the relative importance of top-down control by predation and herbivory vs. bottom-up control by nutrients and recruitment driving food web dynamics. Rather than arguing which of these forces are more important, we can examine the relative importance of each and how these forces interact to influence food web dynamics. Structural Equation Modeling (SEM) or path analysis is a multivariate technique that can test for the nature and magnitude of direct and indirect effects of multiple interacting factors. SEM is an approach that interprets information about the observed correlations among the traits of organisms or groups of organisms in order to evaluate complex causal relationships. It is a rich technique that is particularly well suited for large-scale observational community or population data sets. Its intuitive connection to how we conceive of our study systems makes it a powerful and useful technique for ecologists and evolutionary biologists. The goal of this workshop is to familiarize ecologists the basic techniques of SEM using the 'lavaan' package in R. Material Covered (note: these will be revised before Sunday) Day 1 - Lectures: What is SEM? How can it be part of your research program? pdf SEM as a process: Creating multivariate causal models pdf Fitting piecewise models pdf Exercises: Creating causal conceptual models Piecewise model creation r file, data Readings: Grace 2010 (note: pdfs password protected, email me for info if you've lost it) Optional Reading: Matsueda 2012 (history) Day 2 - Lectures: Fitting Observed Variable models with covariance structures pdf What does it mean to evaluate a multivariate hypothesis?pdf ANCOVA revisited & Nonlinearities pdf Exercises: Fitting observed variable structural equation models in R New R Files and Data, model averaging script Readings: Grace and Bollen 2005, Shipley 2004 Optional Reading: Pearl 2009 Day 3 - Lecture: Multigroup models pdf Latent Variable models pdf Exercises: Multigroup analysis and the introduction of the latent variable R Files and Data Reading: Mancera et al. 2005 Optional Reading: Grace and Jutila 1999, Bollen and Pearl 2012 Day 4 - Lecture: Composite Variables pdf Revisiting piecewise approaches for nonlinear and hierarchical data pdf How to Fool Yourself with SEM (sensu Kline) pdf Exercises: Composites & Other Advanced Techniques R Files and Data Reading: Grace et al. 2010, Shipley 2009 [R appendix]Day 5 - Presentations and/or Open Consultation Optional Reading: Pearl 2012 Before the ClassHello everyone! I'm looking forward to our upcoming journey into the wild world of Structural Equation Modeling together. Before the class begins, I want to make sure you all are prepared so that you can get the most out of it and to have you work through this preclass-excercise and tutorial. The course is going to be a mixture of lecture and hands-on exercises. Aside from model building and identification exercises, everything else will be in R. I am going to assume that you have at least basic proficiency in R. You will need basic proficiency in order to work through the the class. What does basic proficiency mean? I expect that 0) You have R installed on your computer and be able to access help files.1) You can install and load new libraries, as shown in the preclass tutorial.2) You are able to load in data files as shown in the preclass tutorial. 3) You can plot a relationship between two variables from your data. 4) You can perform a basic linear regression, and get summary statistics for parameter fits and F-tests. If you cannot do these things, I encourage you to visit one (or more) of the following additional tutorials (I'm listing several, as I've found that different tutorials work more or less well for different people - so take a look at them and see what works for you.) http://www.r-tutor.com/r-introduction http://www.nceas.ucsb.edu/files/scicomp/Dloads/RProgramming/BestFirstRTutorial.pdfhttp://www.youtube.com/user/wildsc0pIf you can do more than what is above, great. But make sure you work through the preclass tutorial.In the tutorial, you'll be asked to install the following libraries, as we'll be using them throughout the course: lavaanplyrggplot2AICcmodavgcarggmsemPlotsemToolslavaan.surveyFor the exercises, we will work on some common datasets. However, I'll also provide some time to begin working with your own data. If you do not have data to work with, don't worry. I have a few public data sets from kelp forests in Santa Barbara and fouling communities in Bodega Bay. There are also some good sets in the nceas data catalog. However, I highly encourage you to bring your own data as you'll get a much deeper understanding working with SEM if you know the data. You're also welcome to share data sets with each other - who knows, someone may figure out that problem you've been hitting your head on for the past few months! If you have any further questions, please don't hesitate to contact me! Software
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