Stanley Smith Stevens defined measurement as ‘the assignment of numbers to objects and events according to rules’ (Stevens, 1951). This book provides an ideal starting point for researchers and clinicians in search of instruments with which to allocate ‘numbers’ in psychiatry.

There is a long an honourable tradition of measurement in psychiatry, since psychiatry has had to think much harder than other specialties about ‘rules’. Psychiatry deals with concepts and phenomena that are important, but not as readily observable or measurable as in other clinical specialties. Notions of validity and reliability therefore need to be examined in some detail before a measurement instrument can be claimed to be a measurement of anything at all. Psychiatry's gift to the rest of medicine has been an appreciation of the importance of psychometrics and the development of some very good instruments. Historically, several instruments have found a place in wider medical practice (such as the Hospital Anxiety and Depression Scale). There are excellent instruments available for most areas of psychiatric practice, and clinicians and researchers are sometimes faced with a dizzying choice. Part of this choice stems from a ‘measurement industry’, whereby new instruments are continually developed and refined. Unfortunately, much of this happens without due consideration of what has gone before, and new is not necessarily better. In addition, some perfectly acceptable instruments are freely available in the public domain, whereas newer instruments sometimes come with an expensive price tag.

A little careful thought when choosing an instrument often saves time and money further down the line. There is a temptation to measure everything that is possible just because an instrument is available. The guiding principles when choosing outcome measures are to pick only those that measure what is important, and to choose instruments that are fit for purpose. Please bear in mind the effort involved in administering and filling in measurement instruments and always remember that ‘less is more’. This book will help you in all these tasks.