A common question I am asked is “Why or How is Psychological Testing Useful in Therapy?”
There is an article about this very issue at: http://en.wikipedia.org/wiki/Psychological_testing
Also, you can read below…
Psychological testing is a field characterized by the use of samples of behavior in order to assess psychological construct(s), such as cognitive and emotional functioning, about a given individual. The technical term for the science behind psychological testing is psychometrics. By samples of behavior, one means observations over time of an individual performing tasks that have usually been prescribed beforehand, which often means scores on a test. These responses are often compiled into statistical tables that allow the evaluator to compare the behavior of the individual being tested to the responses of a norm group.
Psychological assessment is similar to psychological testing but usually involves a more comprehensive assessment of the individual. Psychological assessment is a process that involves the integration of information from multiple sources, such as tests of normal and abnormal personality, tests of ability or intelligence, tests of interests or attitudes, as well as information from personal interviews. Collateral information is also collected about personal, occupational, or medical history, such as from records or from interviews with parents, spouses, teachers, or previous therapists or physicians. A psychological test is one of the sources of data used within the process of assessment; usually more than one test is used. Many psychologists do some level of assessment when providing services to clients or patients, and may use for example, simple checklists to assess some traits or symptoms, but psychological assessment is a more complex, detailed, in-depth process.
Typical types of focus for psychological assessment are to provide a diagnosis for treatment settings; to assess a particular area of functioning or disability often for school settings; to help select type of treatment or to assess treatment outcomes; to help courts decide issues such as child custody or competency to stand trial; or to help assess job applicants or employees and provide career development counseling or training.
A useful psychological measure must be both valid (i.e., there is evidence to support the specified interpretation of the test results) and reliable (i.e., internally consistent or give consistent results over time, across raters, etc.).