Published on *OpenMx* (http://openmx.psyc.virginia.edu)

By *JWiley*

Created *10/31/2011 - 04:38*

Mon, 10/31/2011 - 04:38 — JWiley [1]

Whenever I try to fit models and add covariances with arrows=2, I always have a heck of a time getting the model past the 'expected covariance is non-positive definite' error. Is this normal? I do not have nearly this much trouble with arrows = 1. Here is a little example using a built in dataset:

#########################

summary(mxRun(m <- mxModel("Example", type = "RAM",

manifestVars = colnames(ability.cov$cov), latentVars = "G",

mxData(ability.cov$cov, type = "cov", numObs = ability.cov$n.obs),

mxPath(from = "G", to = colnames(ability.cov$cov)),

mxPath(from = colnames(ability.cov$cov), arrows = 2),

mxPath(from = "G", arrows = 2, values = 1, free = FALSE))))

summary(mxRun(mxModel(m, mxPath(from = "vocab", to = "reading", arrows = 2, values = 41))))

summary(mxRun(mxModel(m, mxPath(from = "vocab", to = "reading", arrows = 2, values = -.1))))

summary(mxRun(mxModel(m, mxPath(from = "vocab", to = "reading", arrows = 1, values = .4))))

summary(mxRun(mxModel(m, mxPath(from = "vocab", to = "reading", arrows = 1, values = 2))))

#########################

even knowing (from the second attempt), that one estimate that works well is 41.579037 and setting a starting value of 41, it runs into problems. Is there a reason it seems more sensitive to start values with covariances than just paths?

**Links:**

[1] http://openmx.psyc.virginia.edu/users/jwiley

[2] http://openmx.psyc.virginia.edu/thread/1153

[3] http://openmx.psyc.virginia.edu/thread/1098

[4] http://openmx.psyc.virginia.edu/forums/openmx-help/openmx-structural-equation-modeling