Eric G. Mart, Ph.D., ABPP | July 7, 2013
The vast majority of psychologists who perform forensic evaluations utilize psychological testing as part of the assessment process. This raises an important question: Do these tests provide accurate information? The answer to this question is, as with most things in psychology, “It depends.”
Psychological tests have a number of measurable qualities that can help us determine whether they provide accurate information. The first of these is reliability. Reliability is a measure of the consistency of test results. For example, if someone is tested and found to have an IQ of 120 today, he or she should have an IQ that is close to 120 six months later if re-tested with the same test. If an IQ test produced widely varying estimates of intelligence over time, it would be unreliable, in the same way that a rubber ruler would be unreliable for measuring distance. There are a number of ways to measure this construct, but a psychological consultant can tell you if a particular test has acceptable levels of reliability.
The second important factor related to test accuracy is validity. The term validity, when used in the context of psychological testing, refers to the extent to which a test measures what it purports to measure. To use the example of an IQ test again, we would expect that individuals with high IQs would perform better in school and produce higher levels of income over their lives, while those with subnormal IQs would have difficulty in school and work in unskilled or semi-skilled trades. In the same way, we would expect that individuals who obtain elevated scores on a measure of depression would exhibit symptoms of depression; if they do not, this may be an indication that the test that was used is not particularly valid.
It should be noted that tests can be highly reliable but have low levels of validity. For example, an IQ test that gives an estimate of intellectual ability based on shoe size would fall into this category. An adult’s shoe size is unlikely to vary (reliability), but it is unlikely to tell you much about the intelligence of an individual (validity). On the other hand, it is very unusual for a test to be more valid than it is reliable.
The final factor to be considered is the fit between the test and the construct being measured. If we look at the issue of parenting ability, some tests will be closely connected to this issue and give useful information, while others may not. For example, if the issue is a person’s propensity to be physically abusive to young children, the Child Abuse Potential Inventory does a good job of identifying individuals with psychological characteristics that might predispose them to be physically abusive. On the other hand, the Rorschach ink blot test would give you far less information about this specific issue, because the constructs measured by the test are much less directly related to the issue at hand.
Given this information, it may be helpful for attorneys questioning experts to ask about the reliability and validity of each and every test utilized by an expert, and to obtain information about tests from a consultant knowledgeable about psychological tests. In addition, it will be useful to question the expert about any studies that indicate that the test that the expert used is valid in relation to the psycho-legal matter at hand.
References
Ackerman, Marc J. Essentials of Forensic Psychological Assessment. New York: John Wiley and Sons; 1999.