What is the one magic ingredient that makes someone a great teacher? It is tricky to pin down.
Some assume it is how a trainee performed at university, hence all the initiatives designed to lure graduates with top-class degrees into teaching. But academic high-flyers are not always the best in the classroom.
So is it how well you relate to the children? Or how much fun and technological excitement you squeeze into your lesson plans? Again, neither of those are guarantees. The young, empathetic Miss Hipster may still face classroom chaos, then watch in bafflement as her pupils line up quietly and eagerly for a lesson with stern old Mr Chalkdust.
Yet attempts to mimic an old-fashioned disciplinarian can backfire too. And do not imagine for a second that the magic ingredient is a long-held desire to teach: at least one study has suggested those who drift into the job actually tend to do better than those who dreamed of standing in front of a class since childhood.
So what do great teachers have that others do not? (We might call it the "X factor" if that phrase hadn't been hijacked to mean "passably good at singing karaoke on TV".)
The answer can be found in the groundbreaking research of New Zealand's Professor John Hattie. His study, first published four years ago, remains the world's most ambitious meta-analysis of what works in schools. Several topics that generally get lots of attention - setting, class sizes, the school's status, even teachers' subject knowledge - turn out to be among the least important factors. In contrast, what makes a huge difference includes formative assessment and pupils' understanding of their own performance.
However, just two months ago Professor Hattie added a new factor to the top five: teacher credibility. How teachers gain that is a trickier matter, but at least we now have a word for the magic ingredient. Expect to hear the word "credibility" repeated a lot in education circles over the coming years.
Michael Shaw is editor of TESpro @mrmichaelshaw
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