A question today was emailed in by a mother about how easy it is to compare standardised scores. Is one standardised score more reliable than another? In the eleven plus tests standardised scores are used to place children into rank order. The children who have scored high enough pass and these children did not quite make it do not pass.
A proper measuring scale has zero as an absolute point. An example of a scale that has equal units - but not an absolute zero - is a centigrade thermometer. The zero is placed at the temperature of freezing water. We can say that the temperature rises as much from 0 to 25 degrees as it does from 25 to 50 degrees. It is impossible, however, to say that we will find the temperature at 50 degrees twice as hot as at 25 degrees.
Strictly speaking the standardised score is standardised against complex criteria. We can’t say, however, that a child with a score of 140 out of 140 is twice as clever as a child with a score of 115. There are limitations that must affect the way we look at standardised scores. A child who passes one eleven plus test with a standardised score of 120 is not all that much better than a child who passes a different eleven plus test with a standardised score of 117.