1.
Structural Equation Modelling
(SEM)
An Introduction (Part 3)
2.
CFA Models: Important Steps
• Model Specification
• Model Identification
• Model Estimation
• Assessment of Model Fit
• Model Re-specification
3.
Step 1: Model Specification
• SEM is a confirmatory technique and it
Needs a model that delineates the relationships among variables
Requires a model that is based on theory (Bollen & Long, 1993)
4.
Step 1: Model Specification
• Exogenous variables
• Variables whose causes are unknown and/or not included in the
model
• Variables that explain other variables in the model (i.e. independent
variables (IVs))
• Endogenous variables
• Variables that serve as DVs in a model
• May also serve as IVs
5.
Step 2: Model Identification
• Model must be specified so that there are enough pieces of information to give unique
estimates for all parameters
• SEM involves estimating unknown parameters (e.g., factor loadings, path coefficients)
based on known parameters (i.e., covariances)
• Identification involves whether a unique solution for a model can be obtained
• Requires more known vs. unknown parameters
• Identification is a property of the model, not the data
Does not depend on sample size
i.e., if a model is not identified, it remains so regardless of whether the sample size is
100, 1000, 10,000, etc.
6.
Step 3: Model Estimation
• Over-identified models have infinite # of solutions.
• Parameters need to be estimated based on a mathematical criterion.
• Goal is to minimize differences between the observed and implied covariance
matrices.
• Process begins with initial estimates- start values.
• Is an iterative process – will stop when a minimum fitting criterion is
reached.
When the difference between the observed and implied covariance
matrices are minimized
7.
Step 4: Assessing Model Fit
• Absolute fit
• Relative (Comparative) fit
8.
Common Absolute Fit Indices
• Model X2*
• Non-significant X2 (p>0.05) indicates good fit
• Root Mean Squared Error of Approximation (RMSEA)
• Acceptable fit < 0.10; good fit < 0.05
• Goodness of Fit (GFI)
• > 0.90 is considered good fit
9.
Common Relative Fit Indices
• Normed Fit Index (NFI)
• Incremental Fit Index (IFI)
• Comparative Fit Index (CFI)
• All range 0-1
• Generally, >0.90 is considered good
10.
SEM Model Fit: Rules of Thumb
• Will often see/hear reference to 0.90 or above indicating acceptable model
fit, for indices such as GFI, CFI, NFI, etc.
Typically cite Bentler & Bonett (1980) for this assertation
• Basis for this is rather thin (Lance et al., 2006)
• What Bentler and Bonett (1980) actually said:
“experience will be required to establish values of the indices that are
associated with various degrees of meaningfulness of results. In our
experience, models with overall fit indices of less than 0.90 can usually
be improved substantially” (Bentler & Bonett, 1980, p. 600).
11.
Step 5: Model Re-specification/Modification
• Goal is to improve model fit – changing the model to fit the data
• Caveats
Modifications are post hoc & capitalize on chance!
• General guidelines
Must be theoretically consistent
Must be replicated with new data
12.
Evaluating Your Model
• Theoretical/clinical meaning
Guiding principle
• Residuals and implied correlations
Discrepancies between sample covariance matrix and those implied
by the model
• Tests of path coefficients
Direction, magnitude
• Absence of numerical problems
Direction and magnitude of residuals
Pattern of standardized residuals (z-scores)
13.
Looking for Online SEM
Training?
Contact us: info@costarch.com
Visit: http://tinyurl.com/costarch-sem
www.costarch.com
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