Although there are many approaches to assessing validity, and myriad terms used to describe these approaches, eventually the situation reduces to two circumstances:
1. Other scales of the same or similar attributes are available. In the situation where measures already exist, then an obvious approach is to administer the experimental instrument and one of the existing instruments to a sample of people and see whether there is a strong correlation between the two. As an example, there are many scales to measure depression. In developing a new scale, it is straightforward to administer the new and old instruments to the same sample. This approach is described by several terms in the literature including convergent validity, criterion validity, and concurrent validity. The distinction among the terms will be made clear in Chapter 10.