Adding Example of Grace and Bollen, Bull. Ecol. Soc. Amer.

jebyrnes · May 9, 2011 · 9dee27b · 9dee27b
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diff --git a/grace_bollen_2005.r b/grace_bollen_2005.r
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+####
+#
+# Example of the direct and indirect effects of elevation on plant cover via fires.
+# Model taken from Figure 2.  In the original paper, this example was used
+# to illustrate the meaning of standardized coefficients in SEM versus multiple
+# regression.  Try using the covariance matrix to createa  correlation matrix
+# and calculate partial and semipartial correlation coefficients yourself.  
+# How do they compare?
+#
+# Grace, J.B. and Bollen, K.A. 2005. Interpreting the results from multiple regression and structural 
+# equation models. Bulletin of the Ecological Society of America. 86:283-295
+#
+# last tested: 5/5/11
+# last lavaan version tested: 0.4-8
+#
+# byrnes@nceas.ucsb.edu
+###########
+
+
+library(lavaan)
+
+GB05Mat<-matrix(c(1006.2, -26.2, -139.4, 3636.3, 
+                  -26.2,   2.722, 13.47,   -170.4, 
+                  -139.4,   13.47, 157.8, -1459.6, 
+                  3636.3, -170.4, -1459.6, 66693), ncol=4)
+
+rownames(GB05Mat)<-colnames(GB05Mat)<-c("Plant.Cover", 
+"Fire.Severity", "Stand.Age", "Elevation")
+
+
+model<-'
+  Stand.Age ~ Elevation
+  Fire.Severity ~ Stand.Age
+  Plant.Cover ~ Elevation + Fire.Severity
+'
+
+fit<-sem(model, sample.cov=GB05Mat, sample.nobs=90)
+
+summary(fit)
+
+standardizedSolution(fit)