Message/Author 

Andy Cohen posted on Thursday, July 12, 2007  8:40 am



I am conducting a 2 level analysis in which I would like to include interactions between two main effect variables in the within portion of my analysis. I am defining the variables using the DEFINE command (e.g. IntA_B = A * B). The underlying variables for the interaction terms need to be group mean centered. I have already specified group mean centering for these variables in the VARIABLE command (as that is necessary for the use of the TWOLEVEL option in the ANALYSIS command, but am wondering if the DEFINE command will use the original or centered form of the variables. Thanks. 


The transformations in the DEFINE command are done before the centering. 


Dear Dr. Muthen, I wonder, whether  as a consequence of the above  the newly defined interaction terms should be listed under the CENTERING command? Does the order of operations (first computing the interaction terms, then centering) affect the results? 


Yes, the order of operations matters. Any transformations using DEFINE should be done first and the data saved. The centering should be done on the saved data. 

C. Lechner posted on Friday, February 17, 2012  6:38 am



Ok, thank you very much for your answer! May I ask two additional questions: #1: I suppose the same would apply to interactions between a latent variable and a manifest variable computed using the XWITH command? I would first compute the interaction, save it, and then center it along with the other variables in the model? #2: Assume I have a multilevel model with two predictors and an interaction between the two on level 1. One of the two predictors that interact have a random effect, the other is treated as a fixed effect. The interaction thus has to be treated as a random effect as well. However, do BOTH predictors that are part of the interaction have to be treated as random, or will it suffice to treat one as random and the second one as fixed (as I would assume)? Technically, both works fine, because in a regression or path model, Mplus will treat these interactions as any other variables. But is it correct? 


1. Using XWITH, you would center the manifest variable in Define say, but do that in the same onestep analysis. The latent variable has mean zero in most models and need not be altered. 2. I don't see that the choice of fixed or random for each of the variables has any implication for their interaction. 


Just so that I'm clear, it seems like there is no way use the CENTER command to group mean center a set of variables and then use them in a DEFINE statement in the same procedure. For example: variable: names = AgencyID Gender Age T employ enroll engage housegb incany totsup infsup formsup anysup anyinf anyform; cluster = AgencyID; missing are all .; usevar = Gender Age T incany formsup Intx; categorical are incany; within = T formsup Intx; between = Age Gender; center = grand mean (Age) group mean (T formsup); Define: Intx = T*formsup; This would compute the Intx variable before group mean centering T and formsup and this isn't what I want. Is there any way around this? The manual states that the CLUSTER_MEAN option also cannot be used with subsequent DEFINE statements. So I guess that leaves me with using the SAVEDATA command to save the group means, then running another procedure using those saved variables to compute the group mean centered values. Save that data for a final time, and run a third procedure calculating the interaction term with the saved, group mean centered values. Is that correct? Or did I add in an extra step somewhere. Thanks for all your help! 


It sounds like you are correct. 


Hi, I am running a twolevel model to test group differences before and after an intervention. I'm entering my own time variable to represent the number of days since baseline (see sample script below). I noticed that Mplus is automatically centering my time variable. Is there a way to not center it? I would like the baseline (T1) to = 0, as this is more meaningful. Thanks! DATA WIDETOLONG: WIDE = DV_T1 DV_T2 DV_T3  T1 T2 T3 ; LONG = DV  timeB ; IDVARIABLE = person ; REPETITION = time ; Variable: Names are ID group DV_T1 DV_T2 DV_T3 T1 T2 T3 ; Usevariables are group DV timeB person ; Cluster = person ; Within time timeB; Between = group ; Missing are .; ANALYSIS: TYPE = TWOLEVEL RANDOM; MODEL: %WITHIN% s  DV on timeB ; %BETWEEN% DV s on group ; DV with s ; 


The REPETITION option assigns to the variable time consecutive numbers starting with zero. 

Melvin C Y posted on Friday, January 25, 2013  4:44 am



In my two level sem model, I specified the following (there''re more variables but I''ll keep it simple)  Within is x1 x2; !very low ICC. % within% WL1 by x1 x2 x3; Dep on WL1; % between% BL2 by x3; X3@0; Dep on BL2;  Fit statistics are fine except for L2 srmr (above1.0). When I add groupmean centering to x1 and x2 in a subsequent run, the L2 srmr improved substantially. What could be the reason for this? Should I center? I placed x1 and x2 at within as their ICCs were very low. Thanks.. 


I would feel more comfortable with other fit indices for twolevel modeling. Stay with chisquare, RMSEA, and CFI. 

Kirill Fayn posted on Thursday, May 30, 2013  12:45 am



Hello, i am trying to run my first MLM on mplus and am having difficulty centring my level one variables. The model and the error is below: USEVARIABLES ARE Interest Cope1 Nov1 ZOpen ZInt; WITHIN = Cope1 Nov1; BETWEEN = ZOpen ZInt; MISSING ARE all (9999); CLUSTER = subject; DEFINE: CENTER Cope1 Nov1(GROUPMEAN); ANALYSIS: TYPE = TWOLEVEL RANDOM; MODEL: %WITHIN% IntCop  Interest ON Cope1; !need to make these factors IntNov  Interest ON Nov1; %BETWEEN% Interest IntCop IntNov ON ZOpen ZInt OUTPUT: TECH8 SAMPSTAT; *** ERROR in DEFINE command Error in assignment statement for CENTER Could you please help. The syntax seems to be right so I am guessing I can't centre these variables for some reason. Thanks in advance for your time. Best regards, Kirill 


What version of Mplus are you using? If it is earlier than Version 7, the CENTERING option was in the VARIABLE command. If it is Version 7 or later, please send the output and your license number to support@statmodel.com. 

Katerina Gk posted on Wednesday, October 09, 2013  4:42 am



Hi, I have 5factor model(job sat.) and selfeff.( 3factor model).I want to aggregate by school the observed variables of the job sat. and selfeffi. in the between level. If I use CENTERING = GRANDMEAN (x) is enough to understand that I need to aggregate at between level?the observed variable are the same in two levels.... Missing are all (999); CLUSTER IS sxoleio; DEFINE: CENTER = e1..a1..(GRANDMEAN) ANALYSIS: TYPE IS TWOLEVEL ; ESTIMATOR = WLSMV; MODEL: %within% er1_w by e1@1... ; er2_w by e7@1... ; a1_w by a3@1 ...; a2_w by a1@1 ...; er1_w ON a1_w; .... %between% er1_b by e1@1...; er2_b by e7@1...; a1_b by a3@1 ... ; a2_b by a1@1 ...; er1_b ON a1_b; .... OUTPUT: standardized; Thank you very much Katerina 


If you want an aggregated variable on the between level, use the CLUSTER_MEAN option of the DEFINE command to create it. See Example 9.1 where using this variable versus a latent variable decomposition of the individuallevel variable is discussed. 

Katerina Gk posted on Wednesday, October 09, 2013  11:55 am



Thank you for your help!! 


I am running the following random intercept multilevel model DEFINE: CENTER IS x (GRANDMEAN); ANALYSIS: TYPE IS TWOLEVEL; MODEL: %WITHIN% y ON x; %BETWEEN% y ON x; Is it correct that in such a situation, where x is also used at the between level, x is basically groupmean centered at the within level although I specified GRANDMEAN in the Define section? Is there a way to overrule this procedure and use grandmean centering at Level 1? Many thanks in advance for your time. Ute 


You can put x on the WITHIN list and then create a clusterlevel variable for x on between using the CLUSTER_MEAN option of the DEFINE command. See Example 9.1. 


Hi, In relation to the issues of centering in multilevel models raised above, I have two questions: 1. If any transformations (e.g., interaction between observed variables) are made before centering, it means that the interaction term does not use standardized scores of the products, which violates a basic requirement for the computation of any interaction term. How can I bypass this problem? Or is it not a problem? 2. When using grandmean centering (or no centering) for my withinlevel predictors, the fit of the model is considerably higher than when using groupmean centering. What could be the reason? Is there a preferable centering method for withinlevel predictors? Thank you very much in advance! Best, Paris 


1. It is standard to center, not standardize, variables before creating an interaction between them. 2. See the Raudenbush and Bryk book. This is a complex topic. 

Zen Goh posted on Friday, August 01, 2014  7:16 am



I'm running a 111 modmed model, and have a generic question about centering. (1) Why do we only center the X but not the M(mediator), as we do using HLM software? In HLM software, X and M are specified as groupcentered predictors, so that only the withinlevel relationships are apparent. In Mplus, I'm only allowed to center X but not M. (see code and error message below) (2) how does the lack of centering the mediator in MPlus affects the results and interpretation? Thanks! 

Zen Goh posted on Friday, August 01, 2014  7:18 am



NAMES = clust Gender Age Child WLoad WFCts WFCemo LSat MgSup; USEVARIABLES ARE WLoad WFCts LSat Gender Age Child MgSup; MISSING ARE ALL (1); WITHIN = WLoad; BETWEEN = Gender Age Child MgSup; CLUSTER= clust; Define: CENTER MgSup (GRANDMEAN); CENTER WLoad (GROUPMEAN); ANALYSIS: TYPE = TWOLEVEL RANDOM; MODEL: *** WARNING in MODEL command Variable on the lefthand side of an ON statement in a  statement is a WITHIN variable. The intercept for this variable is not random. Variable: WFCTS *** ERROR in MODEL command Withinlevel variables cannot be used on the between level. Withinlevel variable used: WFCTS *** ERROR in MODEL command Withinlevel variables cannot be used on the between level. Withinlevel variable used: WFCTS *** ERROR in MODEL command Withinlevel variables cannot be used on the between level. Withinlevel variable used: WFCTS *** ERROR The following MODEL statements are ignored: * Statements in the BETWEEN level: WFCTS ON MGSUP WFCTS ON GENDER WFCTS ON AGE WFCTS WITH S1 LSAT WITH WFCTS 


Variables on the BETWEEN list cannot be used in the within part of the model. They are measured on the cluster level. This is not related to centering. Read Example 9.1. It goes over all of the multilevel options. Example 9.2 shows a random slope model with a crosslevel interaction. 

Zen Goh posted on Friday, August 01, 2014  12:53 pm



I think I might not have been clear in my question. My mediator (not moderator) variable is also a within, level1 variable  why do we not center this as well? Would this not affect the interpretation of the results? 


You can center mediators as well. The error message refers to something else. 


Hi, I am running a 111 path model (using manifest variables) with nonindependence in my DV (high ICC1; performance rated by common supervisor). In reading about centering (e.g., Enders & Tofighi, 2007), it is clear I need to groupmean center my predictors to prevent betweengroup variance biasing my results. However, I wonder why we don't groupmean center the DV to "purge" out betweengroup variance in the DV and only include withingroup variance? In books and in papers I generally find that the DV is not centered, but in the case of nonindependence, doesn't the DV then include a lot of "noise" and how exactly does Mplus deal with the DV then? E.g., is defining the DV at the WITHIN level an option? Are there other options to take care of this? Thank you very much for your advice, Tina 


You model betweengroup variance in multilevel modeling. There is no need for anything else. 


Dear Muthén, I appreciate the new define functions in version 7.2, where order of commands has significance. In a multilevel regression model where y(within) is regressed on x1(groupmeancentered (GMC)) and x2(GMC), should the interaction term for x1 and x2 (x1*x2) be computed before or after the GMC? That is, to first calculate x1*x2, and then GMC x1, x2, and x1*x2, OR calculate x1(GMC) and x2(GMC), and then calculate x1*x2 as x1(GMC)*x2(GMC). 1) y(within) on x1(GMC) x2(GMC) x1*x2(GMC) OR 2) y(within) on x1(GMC) x2(GMC) x1(GMC)*x2(GMC) (say that, for example, y is lung cancer, x1 is smoking, and x2 is working with asbestos). Hope you can help me with this conundrum, Eivind Ystrom 


You can get wider input on general centering matters from Multilevelnet. 

Angela posted on Friday, October 09, 2015  1:55 pm



Hello, I am running a model using type=complex and I would like to look at some interaction effects. I read on the discussion board that when running a twolevel model, centering should occur after creating the interaction variables in the define statement. However, I am not looking at effects across levels, so I was not sure if this applied to my model. At what point should I center my variables when looking at interaction effects in a multilevel model? Thanks! 


Type=Complex is not a 2level model but a singlelevel model. With singlelevel models it is not required to center variables, but if it is done it should be before creating the interaction. 

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