Q.Explain, with an example, why the 'lack of a true zero point' is considered a limitation of psychological measurement, and how it affects the interpretation of test scores.

Unlike physical measurement, which starts from a true zero, psychological measurement has no true zero point, meaning scores can only ever be interpreted relatively. …

The lack of a true zero point is considered a genuine limitation of psychological measurement because, unlike the physical sciences, psychology has no natural, objective starting point from which to measure a trait.

In the physical sciences, a quantity like length has a true zero, a table can genuinely have zero length (though none do in practice), and measurement starts meaningfully from that true zero. In psychology, however, there is no equivalent true zero for a trait like intelligence: no person has zero intelligence, since every person possesses some degree of it, however small. Because there is no natural zero to measure from, psychologists must arbitrarily fix some point on a scale as a working 'zero', purely for measurement convenience, not because it represents a true absence of the trait.

This has a direct consequence for how test scores must be interpreted: psychological scores are never absolute quantities, the way a physical measurement is, they only carry relative value, meaningful only in comparison to other scores (such as the scores of a norm group) rather than as a stand-alone amount of the trait being measured. For example, if a student scores 110 on an intelligence test, this number is meaningful only relative to the norms for that test (it shows the student is somewhat above the average, typically around 100), not as an absolute quantity of intelligence in the way that 30 kilograms is an absolute quantity of weight. …