In neuropsychological assessment, understanding how to interpret cognitive test results is crucial to providing meaningful recommendations for patients and referrers. A key distinction in neuropsychology—compared to many other areas of psychological assessment—is the emphasis on comparing an individual’s current cognitive abilities to their premorbid (pre-injury or pre-illness) level of functioning, rather than solely to population norms. This approach is fundamental to accurately identifying acquired cognitive impairment and supporting rehabilitation or other clinical decisions.
Why Not Just Use Population Norms? #
The Role of Norms in Neuropsychological Testing #
Neuropsychological tests are standardised on large populations, providing normative data that show how an “average” person of a particular age group, education, or background might perform. These norms offer reference points, describing test scores in relation to a population mean (average) and standard deviations from that mean:
- Norms are useful for identifying where someone falls compared to the general population, e.g., “average,” “mildly impaired,” or “superior.”
- They guide the initial interpretation of test scores and help to spot clear outliers.
Limitations of Sole Reliance on Norms #
However, relying purely on normative data can be misleading—especially when the aim is to determine change due to brain injury or illness:
- Norms do not capture personal baselines: Some individuals have always functioned above (or below) the norm. A decline may be substantial for them, even if their current scores remain in the “average” range.
- Not all impairments are relative to the norm: Someone can show a dramatic personal decline, but still score within normal limits, making disability invisible without premorbid comparison.
The Central Principle: Individual Versus Their Premorbid Self #
What Is Premorbid Functioning? #
Premorbid functioning refers to the level of cognitive, academic, or occupational ability a person possessed before the onset of brain injury or illness. In neuropsychological assessment, the central question is: Are there abilities that have been lost or disrupted compared to what was previously achievable?
How Is This Assessment Made? #
- Historical information: Educational and occupational history, previous academic performance, and reports from family or the individual themselves.
- ‘Hold’ tests: Measures of abilities relatively resistant to brain injury (such as vocabulary) are sometimes used to estimate prior functioning.
- Test profile analysis: Marked disparities between cognitive domains, when atypical, can indicate acquired deficits.
Example #
A carpenter with a history of above-average reasoning suffers a mild brain injury. Post-injury, his scores now sit at the population average. Norms alone might suggest he is fine, but compared to his own baseline, this constitutes a disabling decline.
Why Is This Approach Crucial? #
- Focus on change: Neuropsychology seeks to quantify what has changed for an individual, not just how they compare with ‘average’ people.
- Functional relevance: Recommendations depend on the person’s prior abilities. For example, whether they can return to work, drive, manage finances, or live independently after injury.
- Syndrome consistency: The pattern of deficits should make sense in the context of the neurological syndrome under investigation.
Integrating Norms and the Premorbid Perspective #
Why We Still Use Norms #
- Baseline for interpretation: Norms provide essential context for all test interpretation.
- Identifying abnormalities: Certain disparities in test scores are only considered ‘abnormal’ if they occur infrequently in the normal population (e.g., fewer than 10% of people).
- Statistical thresholds: Base rate statistics help identify statistically meaningful changes, supplementing the premorbid approach.
Balancing Both Approaches #
A best-practice neuropsychological assessment integrates both:
- Reference to norms: To measure how unusual a score or profile is statistically.
- Premorbid comparison: To determine whether an individual has acquired an impairment relative to their own previous abilities.
Summary #
For clinical neuropsychologists, the primary goal is not merely to compare a person to the ‘average’ population, but to detect and quantify cognitive changes relative to their premorbid self. Population norms remain crucial for interpretation, but only in the context of the individual’s own history, abilities, and the clinical question at hand. This approach ensures that recommendations are tailored, accurate, and truly meaningful for the person being assessed.
