Correlating slope and intercept PreviousNext
Mplus Discussion > Multilevel Data/Complex Sample >
 Rich Mohn posted on Sunday, August 11, 2013 - 5:10 pm
Hello, the model below has random slopes and intercepts for the variable surgery . . . I'd like to correlate the intercepts and slopes for surgery. Any help would be appreciated.

CLUSTER = clinic;
WITHIN = base_qol surgery;
post_qol ON base_qol;
s | post_qol ON surgery;
 Linda K. Muthen posted on Monday, August 12, 2013 - 9:46 am
You would say:

post_qol WITH s;
 Rich Mohn posted on Monday, August 12, 2013 - 10:23 am
Thank you
 Max Nachbauer posted on Tuesday, February 21, 2017 - 7:06 am

I am running a multilevel model with random intercepts and random slopes. The dependent variable is math achievement, the independent variable SES.

The syntax is the following:
NAMES = idc math ses;
CLUSTER = idc;
USEVARIABLES = math ses;
WITHIN = ses;


TYPE = twolevel random;

slope | math ON ses;
math WITH slope;

If I calculate the correlation between intercepts and slopes using the covariance of intercepts and slopes, the variance of the intercepts and the variance of the slopes (all in the regular Mplus output), my result is r = 0.214

If I save the values of the intercepts and slopes (using save=fscores) and then calculate the correlation between intercepts and slopes, my result is r = 0.357

What is the reason for the different values of the correlation?

Which correlation should be used?

Thank you in advance!
 Bengt O. Muthen posted on Wednesday, February 22, 2017 - 11:56 am
Estimated factor scores don't behave quite like the true scores - see e.g. our FAQ

Factor scores

You should use the model-estimated correlation.
 Max Nachbauer posted on Thursday, February 23, 2017 - 7:06 am
Thank you for your answer!

Allow me to raise one follow-up question.

If I got your point right, your explanation refers to the latent modeling of intercept and slope used in Mplus. To check that I ran the same model in HLM and also saved the values of the intercepts and slopes and correlated them. The results are quite similar: the correlation within the model is r = 0.221, the correlation using the saved values is r = 0.376

Therefore, I assume that the difference between the two correlations is not due to the difference between true factor scores and estimated factor scores. Rather, I assume that the reason is connected with multilevel models in general.

What do you think?
 Bengt O. Muthen posted on Thursday, February 23, 2017 - 6:20 pm
I don't see how you reach your conclusion or what you mean by the difference being connected with multilevel models in general. The difference between model-estimated and factor score estimated correlations is well known in the literature as our FAQ points to. That's the whole story.
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