Wednesday, August 13, 2014

Plotting mixed-effects model results with effects package

As separate by-subjects and by-items analyses have been replaced by mixed-effects models with crossed random effects of subjects and items, I've often found myself wondering about the best way to plot data. The simple-minded means and SE from trial-level data will be inaccurate because they won't take the nesting into account. If I compute subject means and plot those with by-subject SE, then I'm plotting something different from what I analyzed, which is not always terrible, but definitely not ideal. It seems intuitive that the condition means and SE's are computable from the model's parameter estimates, but that computation is not trivial, particularly when you're dealing with interactions. Or, rather, that computation was not trivial until I discovered the effects package.

Tuesday, August 5, 2014

Visualizing Components of Growth Curve Analysis

This is a guest post by Matthew Winn:

One of the more useful skills I’ve learned in the past couple years is growth curve analysis (GCA), which helps me analyze eye-tracking data and other kinds of data that take a functional form. Like some other advanced statistical techniques, it is a procedure that can be done without complete understanding, and is likely to demand more than one explanation before you really “get it”. In this post, I will illustrate the way that I think about it, in hopes that it can “click” for some more people. The objective is to break down a complex curve into individual components.