# why is -2LL value of nested model smaller than that of full model, and how to find the best start value

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Joined: 05/19/2010 - 05:43
why is -2LL value of nested model smaller than that of full model, and how to find the best start value

question1：
when I simulated the model, I found the -2LL value of some nested models (which had less estimate parameters than full model) were smaller than that of full model, without any "warning" in script. I think it is abnormal, and I have no idea why it happened, and could you give me some information or suggestion to solve this problem.
Thank you!

question 2：
In Mx, there are some order to help me to find the best start value, like "option THard=-n" or "option jiggle", so, is there any order in OpenMx with the same function. Could you give any clue?
Thank you again!

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Joined: 07/30/2009 - 14:03
I suspect that these may

I suspect that these may actually be one question. Unexpected -2LL may occur if the optimizer is stuck in a local minimum. Trying again with several starting value is one way to see if this is the problem.

One way to use R to generate your starting values might look like this:

rstart <- rnorm(1, mean=0, var=.5)
somePath <- mxPath(from=x, to=y, values=rstart, free=TRUE)

You can use R variables in specifying values= arguments in mxMatrix and mxPath.