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Understanding Psychological Testing

©Vicky Campagna, PhD

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          Non-psychologists are often unnecessarily intimidated by psy­chological testing.  They believe that psychologists can administer tests that will provide a veritable window into their unconscious and reveal all their darkest secrets.  Test results are too often accepted uncritically as being 100% accurate, when in fact, all too often there are serious errors that can impeach the test’s conclusions.

The usefulness of any psychological test in the forensic arena is largely dependent on two things: reliability and validity.  A test is considered reliable if test results from the same person remain con­sistent (within a small range of error) over time.  If a test wasn’t reli­able, we wouldn’t know which result was in fact accurate: was it the result from the 1st administration of the test or was it the result from the 2nd administration?  Or neither?  Now, this presumes that we’re talking about a relatively short period of time (except for IQ tests, where the score should remain in the same general range throughout one’s lifetime).  If, for instance, you take the MMPI in August, we’d hope to get the same general results[1] as if you took it in September.  The more consistent a test is, the more reliable it is considered to be.

          But reliability isn’t worth squat until we look at the test’s valid­ity.  Validity refers to the degree to which a test measures what it purports to measure.  For instance, an IQ test is valid for measuring intelligence---that is, it measures the subject’s level of intelligence, but it is not valid for assessing someone’s personality. 

          How do we know that a test is valid?  A commercially-sold test (which are the only kind that should be used forensically) should have reliability and validity information as part of the test manual.  (Beware!  There are some so-called “tests” that do not have this data.)  Most commercially-sold tests are investigated intensely and the results used to establish test norms, i.e. statistics that show how people similar to the person being currently evaluated performed on this test.  The normative group should be identical on all factors of importance (which will vary from case to case) as the client being evaluated.  For instance, there are specific norms for child custody litigants on the MMPI.  If, for example, a custody litigant’s MMPI scores are com­pared to the general population (and not to child custody litigant norms), the results will not be valid.

          The concept of validity is far more complicated that can be dis­cussed in this brief paper.  Suffice it to day, there are many different components that make up test validity and how it is measured.

          Validity and reliability aside, there are different kinds of psych-ological tests.  There are intelligence tests, which measure some­one’s intellectual ability or IQ.  There are also innumerable kinds of personality tests. Many, like the Minnesota Multiphasic Personality Inventory (MMPI) are “self-report” tests, in which the subject simply reports his responses, often in a true/false format.  There are also projective tests, which purport to assess some aspect of a subject’s personality by the way s/he responds to an ambiguous stimulus.  The Rorschach Ink Blots are perhaps the best known (but certainly not the only) example of a projective test.  The subject is shown an ambigu­ous stimulus---in this case, the ink blot---and inferences about his personality are made from what s/he says s/he imagines it might be (a.k.a.  what s/he “projects”).  Since the blot is, in fact, nothing at all, the theory is that what the subject chooses to “see” there (a process known as “projection”) says something about his or her personality. 

          There is disagreement amongst psychologists as to whether or not the entire “projective hypothesis” is even true!  And, as you may imagine, it is considerably more difficult to establish reliability and va­lidity data for projective tests, and for this and other reasons, their use in a forensic arena is highly controversial. 

Not every forensic mental health question necessitates the use of testing, by the way, and it is also true that testing potentially can be challenged by the astute attorney who has taken the time to under­stand its nuances.  At the same time, appropriate testing can fre­quently shed light on previously poorly-understood behaviors.

          When should you consider requesting psychological testing? Whenever your client’s mental health is part of the legal case, testing is one option that should at least be considered.  Here are some questions to ask the psychologist before authorizing a test battery:

1)     Why is testing needed in this case?

2)     Which tests do you propose to use?

3)     For each test:

A)   what are the reliability and validity statistics?

B)   how closely does my client resemble the normative group?

4)     On what grounds could this test be challenged by opposing counsel?

Testing can be a valuable adjunct to any litigation.  But it is also a potential minefield for the attorney who is unfamiliar with its specific caveats and limitations.  It can provide valuable, empirically estab­lished support for your position, or it can torpedo the best-laid strategy.  I am available to help you critically examine the role of psychological testing in your case. And, as always, the first half hour of any consultation is absolutely gratis!  Call me at (650) 368-8318.


[1] On a test like the MMPI, the scores from 2 different administrations will likely not be identical, but they should be in the same approximate range.  The pattern of scores should also be roughly similar.


Vicky Campagna, LMFT, PhD
165 Arch St.,
Redwood City CA 94062
Phone: 650-368-8318
Fax: 650-679-9378