Throughout this book, we have used lots of different test and measurement instruments: Intelligence tests, the Posttraumatic Stress Disorder (PTSD) Checklist, the Asian Values Scale, or the Semi-Structured Clinical Interview for Diagnosis (SCID). Psychological instruments do not fall from heaven with an instruction manual. All psychological tests have to be created and developed through the painstaking work of researchers. During the development process, especially toward the end, a number of statistical tests are run to investigate how the instrument functions. The new instrument goes through a series of test runs, with test samples, to see if it works as it was designed to work. Unit VII covers some of the most important statistical procedures that are part of this process.