Get [M]oving with Mplus – part 2: A Serving of Commands with a Side of Syntax

You know the saying, “it’s not on the outside that matters, it what’s on the inside that counts”? Well, that’s certainly the case with Mplus. It’s time to get a taste of what Mplus really has to offer with it’s very simple and intuitive syntax.

A good place to start is a blank slate. When you open up a new syntax window, that’s what you get. Oftentimes I’ll copy an old syntax and just replace the details, but it’s typically a wise decision to get a sense of the required syntax and how it works so that you can quickly troubleshoot when the time comes when Mplus refuses to run.

Let’s get started with a few general notes (albeit very important notes – details matter in Mplus!):

• All command headings/lines must be followed by a colon (:) and will turn blue
• Subcommands and syntax lines must be followed by a semicolon (;)
• Use an exclamation point (!) to make short notes that will turn green and Mplus does not read (note: if your note is not green, this means you forgot the exclamation point!)

Next, I’ll go through the command lines and their respective subcommands.

TITLE: ! command allows you to name or label what you are testing

Simply the best title ever;  ! This is an example title

! Helpful for keeping track of multiple analyses, although I often forget to change this!

DATA: ! command provides info about the dataset to be analyzed

! I will have a detailed post on getting your data into Mplus, but here are a couple helpful notes:

! Two types of data can be entered: individual data and summary data (see type subcommand below)

! Store data in the same folder as the Mplus syntax (for ease, otherwise must specify full path to file)

! Data Subcommands:

File is FILENAME.dat; ! Tells Mplus where the data file is stored

Type = !means stdeviations correlation; !can import individual or summary data, default is individual

Nobservations = !___; !this command tells MPlus # of observations when type = summary data

VARIABLE: ! command provides info about the variables in the dataset to be analyzed

!Names of variables MUST NOT exceed 8 characters

! Variable subcommands:

names are ! list your variables here; !this tells MPlus which variables are in the whole data set – you could also use the subcommand “Names=”

categorical are ! X1; ! describes type of variable, Mplus assumes variables are continuous unless told otherwise, can specify categorical, nominal, count, etc.

missing are all(-999); ! Must replace missing data with identifier (e.g., -999) before importing data

usevariables = ! list variables you want to use in analysis here;

!If variables are sequential you can simply place a hyphen between the first and last variables (e.g., for X1, X2, X3, X4 you can put X1-X4)

!If variables are created in define command below, add them to END of usevariables list

DEFINE: ! command used to transform existing variables and create new variables

! Define subcommands: you can…

! create new variables, “X12 = (X1 + X2)/2;” OR “X12 = mean(X1 X2);”

! create interaction terms, “int = X1*X2;”

! provide conditional statements, “IF (X1 EQ 1) THEN group = 1;” “IF (X1 EQ 2) THEN group = 2;” Mplus creates variable “group”

! make transformations,

! create parcels, X = mean(X1 X2 X3)

! center variables, “Center X1 X2 X3 (grandmean);”

ANALYSIS: ! command used to describe the nature of the analysis

! Analysis subcommands:

! Type = twolevel; ! nature of the model, default is general for typically don’t have to specify unless doing multilevel analysis (for SEM, regression)

! Estimator = ML; ! Tells Mplus which estimator you want to use, such as ML or Bayes

! Bootstrap = 10000; !Tells Mplus the number of bootstrap samples you want

MODEL: ! command used to describe the model to be estimated

!Three Fundamental commands:

! “ON”

! used to specify regression path (B matrix)

! Y on X1 X2; !regress Y on X1 and X1 or X1 and X2 predict Y

! “WITH”

! Used to specify covariance/correlation *double-headed arrow

! X1 with X2 !can be covariance between observed or latent variables

! “BY”

! Used to specify factor loadings

! X by X1 X2 X3;

! By default, first loading is fixed to 1.0 (similar to other programs)

!Some other things you can do under the model command:

!Constrain a parameter

! Y on X1@.25; ! Fixes regression weight of X1 to .25

! [X1@0]; ! Constrains X1 intercept to 0

! Label or name parameters/paths:

! M on X (a);

! Y on M (b);

! Y on X (c);

!These paths can then be used later for fun things in the model constrain command

!Constrain effects of IVs on DV to be equal:

!Y on X M (a);

MODEL CONSTRAINT: ! Command used for applying your labelled parameters

! Example: mediation using labelled parameters above

new (med total);

med = a*b; 

total = a*b+c;

MODEL INDIRECT: ! Command designed for testing mediation effects

! Mplus provides total and indirect effects (similar to PROCESS macro in SPSS)

! Combine with bootstrap option in analysis to get bootstrapped indirect effects

! If using Bayes estimator, do not use this, but rather use model constraint above

OUTPUT: ! Command used to request additional info not included in default

! Put all desired commands into a single line of syntax

! Popular output subcommand examples:

! standardized(all); !provides standardized estimates and standard errors

! sampstat; !provides sample statistics (e.g., sample means, (co)variances, correlations)

! modindices(all); !for SEM analyses, provides modification indices for model fit

! residual; !requests residuals for the observed variables in the model

! CInterval; request confidence (bootstrap)/credibility(HPD) intervals

! Tech1-16; Requests a variety of additional info on analysis

! Tech1; parameter specification and starting values

! Tech3; request estimated covariance and correlation matrices

! Tech4; means, covariances, correlations for latent variables

! Tech8; requests the optimization history in estimating the model (shows how long the analysis takes)

! See p.713+ of Mplus manual for other output subcommands

SAVEDATA: ! Command saves sample correlation and covariance matrices in separate ASCII file

! Savedata subcommands:

FILE IS output.sav;

! SAMPLE is output.sav; !speficies file name for sample statistics to be saved

! RESULTS ARE output.sav; !specifies name of file in which results of an analysis will be saved

!See p.744+ of manual for all save subcommands

PLOT: ! Command used to request graphical displays of observed data and results

! Plot subcommands:

! Type=PLOT1-3; used to specify types of plots (3 settings)

! PLOT1; see p. 763 in manual

! PLOT2; see p. 764

! PLOT3; see p. 765

!plot command does not work on Macs yet unfortunately, but I do have a how to guide for plotting Mplus outputs in R

So those are essentially the basic commands, subcommands, and a little bit of syntax. It may seem like a lot, but rarely will you need very much to run a single analysis. Here is an example of plausible input with a factor loading and a latent factor predicting an observed factor and a copy-and-paste basic template:

TITLE:
ENTER TITLE HERE;

DATA:
File is ENTERFILENAMEHERE.dat;

VARIABLE:
names are ENTER VARIABLE NAMES HERE;

missing are all(-999); !or whatever your missing value ID value is

usevariables = ENTER VARIABLES TO BE USED;

ANALYSIS:
estimator = ML;

MODEL:
ENTER YOUR DESIRED ANALYSIS HERE
! ON for regression
! BY for factor analyses
! WITH for correlation

OUTPUT:
Standardized Sampstat;

Seems pretty straight forward, eh?