Inspired by the new features in mplus version 6 I am trying to estimate a multilevel latent covariate model using Bayes estimation in a sample of 192 individuals in 32 teams. I have a couple of questions regarding this analysis.
a) Is there a way to obtain any fit statistics for this model using Bayes?
b) In a multilevel CFA with three factor indicators I get abnormally large variance estimates for the between level factor under Bayes but not under MLR when centering (grandmean) the factor indicators. The input I use is:
Centering = GRANDMEAN(Factor1 Factor2 Factor3);
ANALYSIS: Type = twolevel; Estimator = bayes; Process = 2; STVAL = ML; MODEL:
I have two short questions with regards to a twolevel analysis using estimator = bayes.
1. Is there a way to get fit statistics for a twolevel bayes analysis? 2. I'm freely estimating the first factor loading of a factor and fixing the variance of the factor at 1. Is it ok to do this for only one factor in the model or do I have to use the same parametrization for all factors?
1. In principle, yes. In current Mplus, no. Compare neighboring models instead.
2. It should be ok in principle, although I don't off hand recall if it violates the current Bayes restrictions on which type of Psi matrices can be handled - try it.
ywang posted on Thursday, September 23, 2010 - 1:49 pm
Dear Drs. Muthen: Parallel growth modeling with Bayesian method for categorical indicator variables (fixed time scores) can not converge even if I increased the integration number to 5000. Here is the error message and input. Any advice? Thanks in advance.
THE CONVERGENCE CRITERION IS NOT SATISFIED. INCREASE THE MAXIMUM NUMBER OF ITERATIONS OR INCREASE THE CONVERGENCE CRITERION.
categorical are bmi1 bmi2 bmi3 stnbdi_b stnbdi_6 stnbdi_8;
Analysis: integration = 5000; ESTIMATOR=BAYES; process = 4; Model: i1 s1| stnbdi_b @0 stnbdi_6 @0.8 stnbdi_8@2; i2 s2| bmi1@email@example.com@2; s2 on i1 ; s1 on i2 ; i1 with i2; s1 s2 i1 i2 on male; s2 on intervention;
The INTEGRATION option should not be used with ESTIMATOR=BAYES; I suspect that you are not actually getting BAYES but MLR.
I would suggest running each process separately as a first step. Then put the processes together. After success with this, you can add the regression among the growth factors and the covariates. If you continue to have problems, send the files and your license number to firstname.lastname@example.org.
Ewan Carr posted on Thursday, December 16, 2010 - 8:11 am
I'm interested in a two-level analysis using the Bayes estimator. It was mentioned above that fit statistics are not currently available for such models in Mplus, and that we should "compare nested models", instead.
1. Is this still the case?
2. How would I compare nested models in this case (with a two level model, using the Bayes estimator)?
1. Yes. 2. We aren't able to do that at this time.
Ewan Carr posted on Wednesday, March 16, 2011 - 12:35 pm
I'm getting slightly confused about the "THIN" option for Bayesian estimation.
I'm running a two-level factor model, using estimator = BAYES. I want to run 50,000 or 100,000 iterations, and then thin the output by 50, giving 1000 or 2000 samples, respectively.
I started out using:
FBITER = 100000; THIN = 50;
Thinking this would achieve the above. However, the model is taking a very (very) long time to run. It then occurred to me that these settings might be causing the model to run for 5 million (50 x 100000) iterations.. which would explain the slow progress.
If that is the case, then to achieve the above I should use:
FBITER = 2000; THIN = 50;
Some clarification about what the FBITER and THIN settings achieve would be much appreciated.
This is correct Ewan. FBITER refers to the actual number of iterations that are recorded, i.e., it does not include those that are thinned out. So FBITER = 2000; THIN = 50; would result in 100000 computed MCMC iterations of which every 50-th is recorded for estimation purposes.
Ewan Carr posted on Thursday, March 17, 2011 - 5:03 am
That would explain why things were taking so long..
For a positive estimate as you have, the p-value is the proportion of the posterior that is less than zero, so it is not a contradiction. From a frequentist point of view you can think of that as the chance that the true value is of opposite sign.
Rob Dvorak posted on Saturday, August 20, 2011 - 6:06 am
First, let me apologize for my ignorance. But it seems a lot of us out here are running into the issue, so I thought I'd post on it. I'm trying to wrap my head around the use of a one-tailed test using estimator=bayes. I would like to use the bayes estimator for the reasons you mention in your intro to bayes paper, however, my working knowledge of bayes is weak. Can you recommend a reading for a scientist (not a statistician) that explains the logic of the one-tailed test in bayes for those of us who are used to two-tailed b/c we're trained as frequentists? I'm sure I will need to justify the use of a one-tailed test in any papers I publish, so a reference (and rationale) for this would be great. Thanks.
Muthén, B. (2010). Bayesian analysis in Mplus: A brief introduction. Technical Report. Version 3.
"The third column gives a one-tailed p-value based on the posterior distribution. For a positive estimate, the p-value is the proportion of the posterior distribution that is below zero. For a negative estimate, the p-value is the proportion of the posterior distribution that is above zero."
So if you get a positive effect estimate, this two-tailed p-value can be seen as the probability that it is negative, that is, that it's not the effect you had expected.
However, I would instead report the more common 95% Bayesian credibility interval (CI) that we show and note if that covers zero or not. But at a first glance, instead of looking for CIs covering zero, the two-tailed p-value is a quick way to scan the results for almost zero p-values - which imply that the CIs likely don't cover zero.
Rob Dvorak posted on Sunday, August 21, 2011 - 4:00 am
I have discovered that in my output, the "estimate" reported, which I had assumed was the mode of the posterior parameter distribution, is slightly different from the mode of the distribution that is shown when I view the posterior distribution graphs (i.e., estimate in output is .27, mode in graph is .31). The 95% CCI bounds, however, are identical in the output and the graphs. Can you help me understand this discrepancy and advice me on which point estimate to report?
Thanks for your answer. I have come across another issue this weekend. When I run the analysis as a standard two level model with the default estimator, I get an error message about a non-positive definite matrix because the model has more parameters than there are clusters. However, when I run the analysis with the Bayesian estimator, I don't get that error message. Is that because it's ok to have more parameters than clusters in a Bayesian two-level analysis? Can I feel comfortable with those results or do I need to reduce the number of parameters?
The missing error message should not be interpreted as an endorsement of one method over the other. ML is entirely based on asymptotic assumptions driven by the number of clusters converging to infinity. Bayes is not but if the number of clusters is smaller than 10 the estimates will be sensitive to the priors.
The error message is based on a technical threshold point - we use the MLF matrix for standard errors but that matrix is singular iff the number of parameters is more than the number of clusters - thus the model behaves somewhat as an unidentified model and our ability to confirm model identification is limited. Bayes doesn't have this threshold as it uses different methods for identifications etc (with Bayes the number of clusters should be bigger than the number of random effects though).
Ok, thank you, that helps. I have 35 clusters so I'm not concerned about sensitivity to priors.
I have also noticed that the error message doesn't appear when I use multiply imputed data, even when using a ML estimator with more parameters than clusters. Is there a reason that more parameters than clusters would be ok with multiply imputed data?
No Lindsay ... but I didn't say it was not ok. We have conducted simulation studies that show that ML works fine even when the number of clusters is smaller than the number of parameters. The error message says that it is not possible to confirm that the model is identified. If you already know the model is identified you should just ignore that warning.
Thanks very much, Tihomir. I've been trying to figure out how to get quantiles of the posterior parameter distributions so that I can determine, for example, what percentage of two parameters' posterior distributions overlap, or where the cutoff is for 80% of the distribution. Can you tell me how I can get quantiles for the posterior distributions? I haven't been able to figure it out from the manual.
So sorry for the multiple posts. Additionally, I am trying to determine whether the model fits better when all the data are analyzed together vs. separate models for subgroups. However, because the GROUPING options is not available with ESTIMATOR=BAYES, I can't figure out how to allow parameters to be estimated separately for groups in the same model so that I can compare the fit to the model with all parameters forced to be equal. Do you have any advice on how I can compare the fit of the subgroup model to the full model?
From my understanding, I don't think I can use MIXTURE because I don't have a latent class. I also don't know how to create a new parameter from the subgroup parameters, because as of now, the only way I can think to analyze the subgroups separately is to use USEOBSERVATIONS and select each group one at a time. Can you elaborate on how I could do the subgroup analysis in one model?
Also, a separate question: I've been trying to figure out how to get quantiles of the posterior parameter distributions so that I can determine, for example, what percentage of two parameters' posterior distributions overlap, or where the cutoff is for 80% of the distribution. Can you tell me how I can get quantiles for the posterior distributions? I haven't been able to figure it out from the manual.
Thank you, I understand now. I apologize for my confusion.
I am still struggling with how to assess model fit. I was hoping to be able to compare the Deviance Information Criterion from the unconstrained model to the model with all parameters constrained to be equal across groups, but with the MIXTURE analysis, the DIC is not appearing in the output. Is there a way for me to get the DIC with TYPE=MIXTURE?
If not, is there another index of fit I could use? The posterior predictive p-value is not showing up in the output, which I am assuming is because I am using TYPE=MIXTURE COMPLEX, but I'm not sure about that.
I apologize, but I am still having trouble understanding your recommendation for determining what percentage of two parameters' posterior distributions are overlapping.
For example, in Group A, the 95% CCI for parameter 1 is (-.12, .29); in Group B, it is (.23, .83), so these intervals overlap between .23 and .29. I would like to know what percentage of the posterior distribution for Group A is >.23 and for Group B is <.29.
I created a new parameter that is the difference of the two groups' parameters. This new difference parameter has a CCI of (.06, .49). However, I don't know how to translate this information into the answer I'm looking for.
Ewan Carr posted on Friday, July 13, 2012 - 2:48 pm
I'm trying to a fit a two-level model with the Bayes estimator.
I have a binary mediator (x3 is binary; everything else is continuous):
x3 ON x1 x2; y ON x1 x2 x3;
y ON w1 w2; x3 ON w1 w2; y ON x3;
Everything works fine, except I'm having a problem with the between-level threshold of the binary mediator (i.e. [x3$1]).
The mixing of the chains for this threshold is really, really bad (traceplot), and the parameters — for both the threshold and the residual variances — tend to infinity (they increase with the number of iterations). Diagnostics for other parameters seem fine.
Is there anything (obvious) I can do about this? Specifically:
Are there any alternative priors that might improve mixing for the threshold?
How important is it to set a binary mediator as categorical?
I would say try this alternative parameterization that eliminates the threshold and instead estimates a mean for X3 via regression on ONE. I cant quite see where the poor mixing comes from but it could be due to small number of between level clusters. You can also run this model with WLSMV estimator and ML, but for ML it is a bit harder to setup. If you want to try changing priors - the place to look would be the variances on the between level - IG(1,1) often improves the situation.
Here are the commands you need for the alternative parameterization (X3 on ONE is minus the threshold)
data: ... variance=nocheck;
variables: usevar= y x1-x3 w1-w2 ONE between=ONE;
%WITHIN% x3 ON x1 x2; y ON x1 x2 x3;
%BETWEEN% [x3$1@0]; y ON w1 w2; x3 ON w1 w2 ONE; y ON x3;
Just realized where the poor mixing comes from -- the threshold parameter is highly correlated with X3_between, which is particularly poor when the size of the clusters is large. The above parameterization should resolve your problem - if not send it to email@example.com.
Ewan Carr posted on Friday, July 13, 2012 - 3:48 pm
That is amazing — the chains are mixing very well now, and the parameters don't tend off to infinity.
You can use TYPE=IMPUTATION with TYPE=THREELEVEL RANDOM. Try running this on the first data set.
Tanja Ka posted on Wednesday, March 06, 2013 - 11:03 am
Hello, I'm estimating a multilevel model with a random slope in Mplus7. When using the default gibbs algorithm, the model converges and everything looks fine. But when I switch to the gibbs (rw) algorithm, I don't get an output for the model. The model converges obviously (as I see by the PSR during the iterations), but the output is just a reproduction of the input file. It happens only in models with random slopes. Could this possibly be a minor bug? I'd like to switch to the gibbs (rw) algorithm to estimate two random slopes in one model. Thanks a lot!
I fitted a two-level model (binary dependent variable) using ESTIMATOR=bayes. I used the default prior, but the bayesian estimates did not resemble the maximum likelihood results (the model was fitted the same way) as they were supposed to. Any idea what could be the reason?
There isn't anything automatically produced. One approach is to use "neighboring" models that are less restrictive. For instance, BSEM can be used to allow residual covariances that can be checked for significance. See the article:
Muthén, B. & Asparouhov, T. (2012). Bayesian SEM: A more flexible representation of substantive theory. Psychological Methods, 17, 313-335.
Linda Guo posted on Sunday, December 01, 2013 - 10:21 am
Hi Linda and Bengt:
I am trying to confirm results of a two-level model with a dichotomized outcome, by comparing estimations from MLWin and that from Mplus. I used Bayes estimation in Mplus and mcmc estimation in MLWin. However, the estimations from Mplus and MLWin appear to be quite different. Below are algorithms I specified in the two software packages.
For MLWin, I specified mcmc(burnin(2000) chain(20000)), and set the starting value to be the estimation from ML methods.
For Mplus, I specified Type = twolevel; Estimator= BAYES; Biterations = 2000; Fbiteration = 20000; and also manually set the starting value to be the estimation from ML methods.
The results of the default ML methods from the two software were the same by the way.I just started on Mplus, and am certainly not familiar with the algorithm used by Mplus. Is there a difference between Bayes estimation and MCMC estimation in the two software packages? Any suggestions on why the results are quite different? Thank you.
Have you checked that the programs use the same model, for instance the same number of parameters and logit/probit link?
MCMC is a technique to get Bayesian estimates that is used by both programs and the two programs should get the same results if set up to estimate the same model.
Linda Guo posted on Monday, December 02, 2013 - 10:07 am
I used logistic regression in MLWin, for Mplus, I specified in the variable section: categorical = my outcome; I first run the ML algorithm and got the same results from both softwares. However after I turn on MCMC in MLWin and Bayes in Mplus, results started to differ.In both sofwares, I have the same number of samples being used in the regression, and the same number of parameters, I'm not sure how to specify the link in Mplus, but I'm assuming after I specify categorical = my outcome, it should run the model as logistic regression? Here's the command I use in to set up model in Mplus: Variable:Names ARE VAR1 VAR2 VAR3 VAR4 VAR5 VAR6;,Missing ARE all (-9999); Categorical = VAR5; Cluster = VAR6; Within = VAR1 VAR2; Between = VAR3 VAR4; Analysis: Type = twolevel; Estimator= BAYES; Biterations = 20000; Fbiteration = 200000; Model: %Within% VAR5 ON VAR1*-0.050 VAR2*-0.175; %Between% VAR5 ON VAR3*0.048 VAR4*0.167; Regardless of whether I specify the starting values to be the ones from ML estimation, or not specify the starting values for each parameter, the results from Mplus are different from those I got from MLWin. Is there a problem in the code I use? Or where else do you think could have gone wrong?
Bayes in Mplus uses probit link; see the Bayes papers we have posted. Mplus does not have logit link for Bayes. So request probit link for your MLWiN run.
ML starting values are not needed for Bayes in Mplus. No starting values need to be given. This avoids high-dimensional integration with ML for models with many latent variables.
Linda Guo posted on Monday, December 02, 2013 - 12:04 pm
I called the probit link in MLWiN,and didn't change anything else (distribution is binomial, number of parameter stays the same), but the results are still different as compared to those from Mplus. I removed the starting values in Mplus, and still didn't get the same results. What else could go wrong here?
this is a question to make sure my conceptual understanding is correct. In models of clustered data estimated using ML or least squared, standard errors tend to be incorrect (underestimated in most cases) due to the non-independence of observations. Thus standard errors are 'corrected' using a sandwich estimator. Am I correct in my understanding that this is not the case in Bayesian estimation because the standard errors and p-values are derived from the posterior distribution of parameters, which is generated using Markov chain Monte Carlo (MCMC). This procedure does not assume that observations are independent. Please correct me if I am wrong. Thanks, 'Alim
Hi, Is it possible to estimate between-level differences in within-level variances using a "random factor loadings" approach in cases where there is only a single outcome/indicator variable? I would like both intercept and variance of the random loading to be freely estimated on the between level (in order to enter between-level predictors). Below is what I tried (without person covariates). To avoid poor mixing, I fixed the residual within-level variance at an arbitrary value >0 (but smaller than the within-variance from a simple multilevel "null" model), and estimated the mean within-person variance (mwvar) as this residual plus the estimated mean loading (squared). The resulting model estimates do not appear completely unreasonable. However, (a) the value for “mwvar” always seems lower when using random loadings than when using a fixed loading, and (b) “mwvar” differs depending on the selected value of the residual variance. Especially (b) makes me believe I am doing something wrong. Your input would be greatly appreciated!
ANALYSIS: estimator = bayes; MODEL: %within% sigma | f by y; f@1; firstname.lastname@example.org; %between% [sigma] (p1); sigma; y; MODEL CONSTRAINT: new (mwvar); mwvar = 0.1 + p1**2;