How to Build a Valid Measure of Behaviour

One of the main problems facing psychology as a science is the issue of validity - what is the relationship between what you measured and what you are actually interested in? One of the things I like about studying movement is how straight-forward this issue is - we're interested in the control of action, so I just measure the action! The most common directly measured kinematic variable is displacement, or position over time; you can then derive (via differentiation) the various rates of change of the previous variable (velocity, acceleration, jerk, and, I kid you not, snap, crackle, and pop). In human movement we never tend to go past jerk, and you can do pretty well with just position and it's rate of change, velocity.

This post will discuss how we start from these basic kinematics and derive a measure of coordination that is  entirely valid, covers the entire space of possible states and provides a unique number for every possible state within that space. Psychology doesn't have a lot of these kinds of variables, but you need to be able to characterise your state space to do the kind of modelling I've been describing and advocating.

Coordinated rhythmic movement. - an introduction to an experimental paradigm

I have some papers I wanted to talk about, but it occurred to me that they only make sense in a context. So I'm going to spend some time talking about the experimental task I use a lot, why it's interesting and what we can learn from it about perception and action.

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To a man with a hammer, everything looks like a nail. My hammer is coordinated rhythmic movement, and I use it to study everything I'm interested in - perception-action, learning, ageing, you name it. This task is a workhorse of the perception-action literature and has been since Kelso described it in 1981 (after the task was, as far as I can find, first described by Cohen, 1971).