**xxM** is a package for multilevel structural equation modeling (ML-SEM) with complex dependent data structures. **xxM** implements a modeling framework called n-Level Structural Equation Modeling (NL-SEM) and can estimate models with any number of levels. Observed and latent variables are allowed at all levels.

## Data structures

- Hierarchically nested data (e.g., students << classrooms << schools << districts).
- Cross-classified data (e.g., students crossed within schools and neighborhoods).
- Partial nesting (e.g., only at-risk students in a classroom receive additional instruction by a tutor).
- Longitudinal data (long or wide).
- Longitudinal data with switching classification (e.g., students changing classrooms/teachers over time).
- Round-robin design (e.g., each person rates every other person in a small group).
- 360 performance evaluation data.
- A combination of above structures for different level pairs.

## Models

- Multilevel models (MLM) with random effects of observed variables (e.g., MLwiN, HLM).
- Multilevel structural equation models (ML-SEM) with observed and latent variables at all levels. (e.g., Mplus, GLLAMM)
- Linear Mixed-Effects Models (LME) with constraints on both
**G**and**R**side of the model. (e.g., lme4:*lmer*, SAS: HPMixed) - Linear Growth Curve (LGC) Models.
- David Kenny’s Social Relations Model (SRM) for reciprocal dyadic ratings.

## Estimation

- Efficient maximum Likelihood (ML) estimation.
- Profile-likelihood confidence intervals (CI).
- Fixed-to-constant and equality constraints on model parameters.
- Designed for very large datasets. Works well with small datasets as well.

## Specification

- ‘If you can draw the model, you can build the model’. Graphical representation of NL-SEM models is intuitive.
- ‘If you can build a toy model, you can build a skyscraper’. Models are constructed using a ‘building-block’ approach.

## Documentation

**xxM**reference manual documents all commands and packaged datasets.**xxM**user’s guide provides examples of NL-SEM models.- Additional examples of ‘live’
**xxM**model fitting sessions are available on the site.

## Support

- Discussion forum.

## Price

- Priceless!

## Ready?

## Acknowledgement

Development of **xxM** was made possible by a generous grant (R305D090024) from the Institute of Education Sciences, U.S. Department of Education awarded to Paras D. Mehta.

## Contact

- paras [dot] mehta [at] times [dot] uh [dot] edu