One of the main problems facing psychology as a science is the issue of

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.

**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.