CoPS FAQs

A1 A child has done Rhymes or Wock and even though they only got one or two wrong the centile is low, why is this? In these assessments in particular it is possible for a pupil to get an unexpectedly low centile score even if he/she only got one or two test items wrong.

Although this may be alarming it is statistically accurate (from the original norming sample) and reflects what is known as a ceiling effect. In simple terms, nearly all of the pupils in the original standardisation sample got all the items right.

The anomaly can be stark in Rhymes as there are as few as 8 test items. As an example, a six year old would achieve a 'perfect' centile score of 99 for getting all 8 correct but only 35 if he/she got just one item wrong. Another example is a 5 year old tested on Wock who is scored 99 centile for all 15 correct but only 32 centile if two items are answered incorrectly. There is usually no need for concern and the low centile can be viewed as a statistical blip.