For Cambridge Esol the key ability, that it believes still eludes computers, is the accuracy by which humans can evaluate the language produced by other humans. But that ability doesn't come naturally, and Cambridge Esol, like other recruiters of examiners, relies on the skills developed by English language teachers and their understanding of the productive and receptive skills of learners.

The list of qualities demanded by Cambridge Esol of its examiners is a long one, but for Chris Betts, assistant director responsible for management of examiners, first and foremost, they need to be able to maintain a consistent quality of assessment.

"They need to analyse candidates' performance against a set of marking scales and identify with accuracy where the candidates sit on the scale. Their primary purpose is to deliver excellent assessment," he said.

To the list he adds administrative skills, interpersonal skills and the ability to respond professionally to feedback – all further qualities of a good teacher.

Most exam providers require their examiners to be educated to degree level, to have a recognised English language teaching qualification and to have a minimum number of years of teaching experience. This can range from the three years required by Ielts to a least five years from Trinity College London.