Abstract
Growth mixture models (GMMs) are applied to intervention studies with repeated measures to explore heterogeneity in the intervention effect. However, traditional GMMs are known to be difficult to estimate, especially at sample sizes common in single-center interventions. Common strategies to coerce GMMs to converge involve post hoc adjustments to the model, particularly constraining covariance parameters to equality across classes. Methodological studies have shown that although convergence is improved with post hoc adjustments, they embed additional tenuous assumptions into the model that can adversely impact key aspects of the model such as number of classes extracted and the estimated growth trajectories in each class. To facilitate convergence without post hoc adjustments, this paper reviews the recent literature on covariance pattern mixture models, which approach GMMs from a marginal modeling tradition rather than the random effect modeling tradition used by traditional GMMs. We discuss how the marginal modeling tradition can avoid complexities in estimation encountered by GMMs that feature random effects, and we use data from a lifestyle intervention for increasing insulin sensitivity (a risk factor for type 2 diabetes) among 90 Latino adolescents with obesity to demonstrate our point. Specifically, GMMs featuring random effects—even with post hoc adjustments—fail to converge due to estimation errors, whereas covariance pattern mixture models following the marginal model tradition encounter no issues with estimation while maintaining the ability to answer all the research questions.
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Notes
Technically, all people are in all classes simultaneously but their contribution to the likelihood of each class is weighted by the probability that they belong to each class. We simplify the description in the text to keep the conceptual idea succinct.
All Mplus input and output files used in the analysis are available from https://osf.io/sjer5/
Based on reviewer comments, we also explore latent basis and multivariate pattern cluster mixtures models to consider robustness of class assignment and trajectories to a quadratic growth function. The results from this exploration revealed that repeated measure means were very reasonably approximated by a quadratic function and that class assignment was not appreciably different among different latent class methods. Full details of this robustness analysis are provided in the appendix.
The number of parameters required for unstructured covariance matrices can be unruly when the number of repeated measures exceeds about 5 (McNeish & Harring, 2020, p. 953). There were no issues in this data containing only 4 unequally spaced repeated measures, so we opted for the most general structure to avoid any potential issues associated with covariance misspecification (e.g., Heggeseth & Jewell, 2013). Readers considering CPGMMs with data featuring more repeated measures are encouraged to consider more parsimonious covariance structures such as Toeplitz, first-order autoregressive, Markov, or first-order factor analytic. More detail on selecting between competing covariance structures in CPGMMs is provided in the appendix.
This interpretation presumes that classes are substantively meaningful entities, typically deemed a direct application of mixture models. It is also possible that the classes are merely a mathematical device to approximate a complex reality that may simply be non-normal (Bauer & Curran, 2003), often deemed an indirect application of mixture models. There is currently no reliable method by which to distinguish direct and indirect applications (Bauer & Curran, 2004). This applies equally to random effect GMMs and CPGMMs.
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Funding
This work was supported, in part, by grants from the National Institute on Minority Health and Health Disparities (P20MD002316 and U54MD002316), the National Institute of Diabetes and Digestive and Kidney Diseases (R01DK107579 and 3R01DK107579-03S1), and the Institute of Educational Sciences (R305D190011).
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McNeish, D., Peña, A., Vander Wyst, K.B. et al. Facilitating Growth Mixture Model Convergence in Preventive Interventions. Prev Sci 24, 505–516 (2023). https://doi.org/10.1007/s11121-021-01262-3
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DOI: https://doi.org/10.1007/s11121-021-01262-3