When you’re next handing out your questionnaire in clinic, or on the wards, or sit out in the library assessing the results of a recent clinical study, will you be asking about the quality of the questionnaire you’re using or reading about?

Of course. I should have realised you will.

Though you might just have a little niggle about what the Cronbach’s alpha actually means …

Well, it’s a measure of internal consistency – that is, an assessment of how much the rating scale in question seems to measure the same thing. It works by comparing the variances of the results of each ‘item’ (test question, often) with the variance of the overall scale, across a population of folk. If the ‘items’ are all measuring the same thing,  then the alpha value is high (generally speaking, >0.7 has been taken as ‘good’ internal consistency).

– Archi