help! Model won't run when applying constraint with multiple operations

[James K]

I cannot for the life of me figure out why my model is not running. I am trying to apply a specific constraint where b4 = -2 * sqrt(b3 * b5).

Here is the model:

      model <- "

      outcome ~ b1*X.c + b2*Y.c + b3*X.c2 + b4*XY

      + b5*Y.c2 

      b4 == -2*sqrt(b3*b5)

      "

Then I play around with the constraint to identify which part of it is screwing things up. The following models run, however:

 

     model <- "

      outcome ~ b1*X.c + b2*Y.c + b3*X.c2 + b4*XY

      + b5*Y.c2 

      b4 == sqrt(b3*b5)

      "

      model <- "

      outcome ~ b1*X.c + b2*Y.c + b3*X.c2 + b4*XY

      + b5*Y.c2 

      b4 == -2*sqrt(b3)

      "

      model <- "

      outcome ~ b1*X.c + b2*Y.c + b3*X.c2 + b4*XY

      + b5*Y.c2 

      b4 == -2*b3*b5

      "

Any insights would be greatly appreciated!

[Edward Rigdon]

Is it possible that b3*b5 is negative?

sqrt(b3) runs, so b3 is positive.

The final estimate of b5 may be positive, but maybe not the starting value or some early iteration.

You could add a positive start value to your first line of code:

      model <- "

      outcome ~ b1*X.c + b2*Y.c + b3*X.c2 + start(0.3)*X.c2 + b4*XY + b5*Y.c2 + start(0.3)*Y.c2

      b4 == -2*sqrt(b3*b5)

      "

Just a thought.

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[James K]

Thanks for your response. 

I was able to resolve the issue by setting the constraint as b4^2 = 4*b3*b5

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