Paradoxically, if you proceed a little further in the search for existing instruments to assess a particular concept, you may conclude that none of the existing scales is quite right, so it is appropriate to embark on the development of one more scale to add to the confusion in the literature. Most researchers tend to magnify the deficiencies of existing measures and underestimate the effort required to develop an adequate new measure Of course, scales do not exist for all applications; if this were so, there would be little justification for writing this book. Nevertheless, perhaps the most common error committed by clinical researchers is to dismiss existing scales too lightly, and embark on the development of a new instrument with an unjustifiably optimistic and naive expectation that they can do better. As will become evident, the development of scales to assess subjective attributes is not easy and requires considerable investment of both mental and fiscal resources. Therefore, a useful first step is to be aware of any existing scales which might suit the purpose. The next step is to understand and apply criteria for judging the usefulness of a particular scale. In subsequent chapters, these will be described in much greater detail for use in developing a scale; however, the next few pages will serve as an introduction to the topic and a guideline for a critical literature review The discussion which follows is necessarily brief. A much more comprehensive set of standards, which is widely used for the assessment of standardized tests used in psychology and education, is the manual called Standards for educational and Psychological tests, published by the American and psychological Association(1974).