Equal threshold intervals in SEM with categorical variables

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tran viet Yen

Feb 21, 2019, 3:09:05 AM2/21/19
to lavaan
Dear all,

When estimating a SEM with categorical variables using, can we constrain the intervals between thresholds? For instance in this simple model with the latent variable A and two indicator A1 and A2:

A =~ A1 + A2
A1 | a1*t1 + a2*t2 + a3*t3 + a4*t4
A2 | a1*t1 + a2*t2 + a3*t3 + a4*t4

The problem is, if I want to place some constraints on the thresholds, for instance to force a2 = -a3 and (a2-a1) = (a4-a3) like in this picture:


then how we will modify the code to obtain this?

Thank you so much for reading this topic.


Tran Viet Yen.

Terrence Jorgensen

Feb 21, 2019, 4:04:10 PM2/21/19
to lavaan
how we will modify the code to obtain this?

You can literally put those in your model syntax, using the "==" operator to define any (in)equality constraint(s) on labeled parameters.  But you need to express the left-hand side as a single parameter, which can be user-defined.

model <- '

A =~ A1 + A2
A1 | a1*t1 + a2*t2 + a3*t3 + a4*t4
A2 | a1*t1 + a2*t2 + a3*t3 + a4*t4

a2 == -1*a3

diff21 := a2 - a1
diff43 := a4 - a3
diff21 == diff43

Terrence D. Jorgensen
Assistant Professor, Methods and Statistics
Research Institute for Child Development and Education, the University of Amsterdam

tran viet Yen

Feb 22, 2019, 1:25:05 AM2/22/19
to lav...@googlegroups.com
Dear Terrece,

Thank you very much for the instructions. This is exactly what I'm looking for. You have explained it in a very detailed manner.

Best regards,

Tran Viet Yen.

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Trần Việt Yên
PhD Student
Civil and Environmental Engineering Department, Graduate school of Engineering, Nagoya University, Japan.
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