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 Robin Martin posted on Friday, August 29, 2014 - 5:19 am
I am doing an 2-1-1 mediation model using Preacher's syntax. I am new to this and wonder can the '2' part of the model be a binary variable (in this case trained vs. not trained) and if so does one need to enter this as a CATEGORICAL variable?
 Linda K. Muthen posted on Saturday, August 30, 2014 - 10:01 am
The 2 part can be categorical. It should not be specified as categorical because it is an observed exogenous variable.
 Robin Martin posted on Sunday, August 31, 2014 - 5:51 am
Many thanks. that is helpful.
Can I please ask about sample size? I suspect you get asked this often!
For the 2 variable I have two clusters (39 vs. 23). However, within clusters there are between 1 and 4 people (average about 3 per cluster). That is because the study involved project groups with maximum size 4. I have seen in a previous message that you recommend including clusters with 1 in (as long as less than 15% sample, as is this case) as it contributes to the between-subject effect. However, what are your views with such a low number per cluster??
 Bengt O. Muthen posted on Sunday, August 31, 2014 - 11:05 am
This need not be a problem. You will see the consequences in terms of the SEs.
 Robin Martin posted on Monday, September 01, 2014 - 2:44 pm
many thanks again.
Wonder if you know of any reference(s) supporting low cluster numbers and inclusion of clusters with one. Just knowing journals like citations to back up things!
 Bengt O. Muthen posted on Monday, September 01, 2014 - 3:27 pm
None comes to mind - maybe check the simulations done by Joop Hox' team, or email him directly.
 Chris Cambron posted on Wednesday, February 01, 2017 - 3:19 pm
Hi Drs Muthen,
Iím working with 2-1-1 models and havenít yet found any examples whereby the Within y ON m is incorporated into the total or indirect effect. Iíve been through Dr. Preacherís papers and the code on his site but could be missing something because I donít think conflation is a concern in my case. Itís meaningful for my analyses to include the Within level y ON m in the mediation model as I expect that the relationship between m and y is not the same for all cases within a cluster. I include some individual level controls that are important for the relationship of m ON x from other multilevel analyses and are as well important for y ON m. Does the following code appropriately allow for this? For context, x is a neighborhood disadvantage, m is parent substance use, y is child substance use, and the controls are family demographics. Thanks in advance!

Within = m controls;
Between = x mmean;

mmean = CLUSTER_MEAN(m);

y ON m (b2);
y m ON controls;

mmean ON x (a);
y on mmean (b1);
y on x (c);

IND = a*(b1+b2);
B = b1+b2;
C = c;
TOTAL = (a*(b1+b2)) + c;
 Bengt O. Muthen posted on Wednesday, February 01, 2017 - 6:09 pm
This looks correct. A somewhat better approach is to use a latent variable decomposition of M (so not just for Y). This means that you don't use mmean and don't put M on the Within list.
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