School tests data 'our best friend' - Prof David Reynolds
Annual school tests for children aged seven to 14 are causing so much stress some pupils are refusing to go to school, a survey of teachers claimed.
Members of the union NUT Cymru said the reading and maths tests put too much pressure on children too young, with one child even stopping eating.
They also raised concerns it affected pupil morale and said the tests had little impact on attainment.
The Welsh government says the tests and the data they provide are fundamental to raising standards, stop pupils falling behind, and have been welcomed by parents.
David Reynolds is professor of educational effectiveness at the University of Southampton, and an adviser to the Welsh government.
He told BBC Radio Wales' Kayley Thomas it was perplexing to hear criticism of tests which look to identify those pupils who may be struggling.
29 Oct 2014