Using SAS PROC NLMIXED to fit item response theory models

181-2007: Using SAS® PROC NLMIXED for Robust Regression

We show how the NLMIXED procedure can be used to fit linear models (simple linear regression, multiple linear regression, analysis of variance and analysis of covariance) when data are not normally distributed or contaminated with potential outliers. This is accomplished using PROC NLMIXED’s capability of fitting user defined distributions through the general log likelihood option in the Model ...

IRT-FIT: SAS® Macros for Fitting Item Response Theory (IRT) Models

Psychometrics has recently seen the development of complex measurement models to better represent test and item data. Item Response Theory (IRT), in particular, comprises a set of non-linear latent variable models that appear to have several conceptual and empirical properties that make them more valuable in practice than classical test theory methods. However, IRT-based models typically requir...

Growing Growth Curves Using PROC MIXED and PROC NLMIXED

Being able to describe growth appropriately and succinctly is important in many contexts, including biology, epidemiology and statistics. Various approaches are used varying from differential equations, deterministic modeling and statistical approaches like regression. Often, with epidemiologic data we want to model growth in the context of demographic variables and other potential mediators. H...

Confidence intervals for the interaction contrast ratio: A simple solution with SAS PROC NLMIXED and SAS PROC NLP

Meta-Analysis with Linear and Nonlinear Multilevel Models Using PROC MIXED and PROC NLMIXED

Meta-analysis is both a theory and a toolbox of statistical techniques for combining summary statistics from similar studies. However, with the notable exception of the FDA New Drug Application, typical studies are not similar. Rather than replicate, they try to extend the generalizability of earlier findings. Consequently, between-studies heterogeneity continues to plague the theory and practi...