Interpreting Psychological and Neuropsychological Tests
Rees Chapman, Ph.D.
July 2011

When I interpret test results in terms of a person's strengths and weakness, I use a system taught me in graduate school.  It is not the only one utilized by psychologists; some assign different levels of impairment and skill to test scores, such that there is no hard-and-fast rule.  Most psychologists with whom I interact, however, use a system like this:

The basic premise holds that increments of impairments and skills can be differentiated by z scores, and that for every z score away from the average (or mean) score the value deviates, another level of function is indicated.  So, any score falling between -1z and +1z is average.  If a test score gets higher as there is more deficit or disorder then z scores above +1z are increasingly impaired.  By contrast, if higher test scores are associated with greater skills, then z scores below
-1z are increasingly impaired.

In this system, the following z score ranges correspond to the following levels of function:

For skills and abilities:

greater than +3z = very superior
between +3z and +2z = superior
between +2z and +1z = above average
between -1z and +1z = average
between -1z and -2z = mildly impaired
between -2z and -3z = moderately impaired
greater than -3z = severely impaired

For impairments:

greater than +3z = severely impaired
between +3z  and +2z = moderately impaired
between +2z  and +1z  = mildly impaired
between -1z  and +1z = average
between -1z  and -2z = above average
between -2z  and -3z = superior
greater than -3z = very superior

Test scores are often expressed in terms other than z scores.  Standard scores average 100, and increase and decrease in 15 point intervals; thus, a z score of -1z equates to a standard score of 85std.  t scores average 50 and increase and decrease in 10 point intervals; a z score of -1z  equates to a t score of 40t.  Percentiles use different intervals than z, standard or t scores; a z score of -1z equals a percentile of 15.9%, while a z score of -2z equates to a percentile of 2.3%.