A scale that gives the same wrong answer every time: reliability without validity on the NCE.
The single most tested idea in the assessment domain, in one practice item. What "necessary but not sufficient" means, the bathroom scale that explains it, and the four validity items that follow from it.
By Dr. Andrew Wichterman · September 22, 2026 · 6 min read
An instrument gives consistent scores across administrations but does not measure the construct it claims to measure. The item asks how to describe it. The keyed answer: high reliability, questionable validity. The sentence that follows is the whole domain in nine words: reliability is necessary but not sufficient for validity.
The practice item discussed here was written for this site in the style of the NCE. It is not a question from the actual exam. NBCC does not release its items, and candidates sign a nondisclosure agreement.
What the item is really testing
Reliability is consistency. Give the test twice, get the same score; split it in half, the halves agree; two raters score it, they match. Validity is accuracy. The test measures what it says it measures and its scores mean what they are supposed to mean. The two are different properties and the exam wants you to keep them apart.
The bathroom scale makes it concrete. A scale that reads five pounds heavy every morning is perfectly reliable. It is not valid. A scale that reads differently every time you step on it is neither: if the readings bounce around, they cannot be tracking your weight. That is the asymmetry the item tests. You can be reliable without being valid. You cannot be valid without being reliable.
Why the distractors tempt
One distractor says the instrument is valid but unreliable, which is the impossible case. Another says it is both reliable and valid, which ignores the stem's "does not measure its claimed construct." A third says neither, which ignores the consistent scores. The item is built so that only one option honors both halves of the stem. Read stems in two parts when they describe two properties; the answer usually has to satisfy both.
The four items that follow from this one
Once the distinction is in place, the domain's validity items are easy. Content validity: do the items cover the domain, judged by experts. Criterion validity: does the score relate to an outcome, concurrent if measured now, predictive if measured later. Construct validity: does the test behave as the theory says, correlating with similar measures (convergent) and not with different ones (discriminant). Face validity: does it look right to the test taker, which is the weakest kind and the one the exam likes to offer as a distractor.
And the reliability items: test-retest for stability, alternate forms for equivalence, split-half and Cronbach's alpha for internal consistency, inter-rater for agreement. The standard error of measurement falls as reliability rises. If you can place a description into one of these boxes, the item is done.
Where this lives in the course
Domain 2, Intake, Assessment and Diagnosis, lesson 4: Reliability and validity. The compare card for reliability versus validity is in the Assessment deck, along with sensitivity versus specificity and norm- versus criterion-referenced.
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Consistent but wrong is reliable but not valid. The reverse is impossible. That asymmetry is the whole item.