July’s over so time to move on from the Determinants of Gait. We’re starting detailed development of teaching material for our new masters degree programme in clinical gait analysis. I’m working on the measurement theory section at the moment and been reflecting how to approach this. I’ve got an engineering background and automatically assume that the language we should use to describe measurement is that of classical measurement theory which I’m going to refer to as metrology.

Modern metrology really started with the French Revolution when a political motivation emerged to standardise measurement systems across the country. Out of this emerged an international process for the standardisation of measurement which is now overseen by the Conference Generale des Poids et Mesures, still based in Paris. They publish the International Vocabulary for Metrology (IVM) which is really the international “Bible” for measurement theory.  The Vocabulary is designed to be universal including the statement that “metrology includes all theoretical and practical aspects of measurement, whatever the measurement uncertainty and field of application.”

One of the things that interests me about measurement in medicine in general and in rehabilitation in particular is that, in some respects, it is developing separately to this paradigm which is accepted almost universally in the physical and biological sciences and in engineering and chemistry. Measurement in medicine and rehabilitation is becoming increasing conceived within the framework of psychometrics. Why, if all the rest of the world is handling measurements one way does, psychology and now rehabilitation need to adopt a different approach?

Whilst its foundations can be traced back to Darwin (see Wikipedia) , psychometrics really came of age in the middle of the twentieth century and is thus a much more recent development than metrology. As the name implies it was developed by psychologists for their work studying concepts such as self-esteem or happiness or even pain which are less specifically defined than quantities in other branches of science. Over-simplifying a bit – metrology was developed to measure things that are specifically defined whereas psychometrics was developed to measure things that are not.

If the quantity you are measuring is specifically defined (e.g. someone’s height) then it is sensible to ask how accurate the measurement is (are you measuring what you claim to be measuring) and this is the fundamental challenge of metrology. If the quantity you are measuring is not specifically defined (e.g. how happy someone is) then the question of how accurate you are is rather meaningless. Psychometry thus focusses on the twin alternative questions of how reliable (repeatable) and valid measurements are.

Others may argue but I am convinced that there is a hierarchy here. If a quantity is well enough defined to determine how accurately it can be measured, then assessing repeatability and validity is second best. If you want to do the job properly you should use metrology to assess accuracy. Psychometric assessment of reliability and validity should be confined to quantities for which the superior option is not possible.

I think that the insidious onset of psychometry has made people lazy. I suspect that there would have been considerably more effort expended on improving measurements in biomechanics if the community had focussed on ensuring accuracy of measurements rather than accepting second best (and rather flattering) measures of repeatability derived from an essentially psychometric approach.

Any volunteers to man (or woman!) the barricades against the insurgence of psychometrics where it isn’t needed or wanted?