Psychologists use tests to measure the nature and extent of individual differences, either numerically or impressionistically. The two major categories of psychological tests are psychometric and nonpsychometric.
To qualify as a psychometric test the scoring procedures must be purely objective, giving the same results regardless of who gives the test. Items on these tests are structured, and the resulting numerical scores yield continuous or discrete frequency distributions that can be analyzed statistically. Examples of this type of test are the California Personality Inventory (CPI), Scholastic Assessment Test (SAT), and Wechsler Intelligence Scale For Children (WISC).
The scoring procedures for nonpsychometric tests involve subjective judgment on the part of the scorers. Items on the tests are unstructured and generally ambiguous. Examples of this type of test are the Children's Apperception Test (CAT), Machover Draw-A-Person Test (D-A-P), Rorshach Inkblots (RI), and Thematic Apperception Test (TAT).
Both types of psychological tests are required by national regulations to comply with the Ethical Principles of Psychologists and Code of Conduct.
Based on: Psychometric Tests. (2004). In Encyclopedia of Applied Psychology.
Here are a few good books to get you started. Use the online catalog to search for others.
Assessment procedures for counselors and helping professionals by 
Encyclopedia of psychological assessment. by 
Psychological testing and assessment / 11th ed. by 
Positive psychological assessment : a handbook of models and measures. by In addition to the specific resources below, the APA's Finding the right tools article is a useful overall guide.
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