Look at the record books and work out how many caretaker coaches are shifted up to the full-time role.
It is not many, so why is it being so widely assumed that Scott Johnson's appointment as interim Scotland head coach is the prelude to offering him the job up to the 2015 Rugby World Cup and possibly beyond?
It happened last year south of the Border when Stuart Lancaster won the England job but, for a start, he got lucky – it is unlikely that he would have kept it beyond the Six Nations last season if Scotland had had an inkling of where to find the try line in his first match in charge when he fluked a win at Murrayfield – and secondly, his position could hardly have been more different to Johnson's.
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Lancaster came to the role as a coach steeped in the strengths and weaknesses of English rugby. He had done his apprenticeship with the academy at Leeds, had spent four years working with the players who are now the core of his national side when he was head of elite player development at the RFU, and had caught up with the rest of the up-and-coming players while coaching the England Saxons.
If the Scottish Rugby Union thought the Lancaster model was the route to success, they would have been persuading Sean Lineen to take the job, since he is the nearest equivalent at their disposal with a background in domestic rugby, developing professional players and a track record of talent spotting.
They didn't. Instead chief executive Mark Dodson, and whoever was advising him on rugby matters – who that may have been is not clear since there is nobody in charge of the rugby section of the union at the moment – went for the outsider.
Johnson may not be totally new, since he has been around for six games over the last six months, but he is recent enough to be a fresh face when he takes charge. He has a reputation for innovative, imaginative thinking and players in all the countries he has worked in – Wales, Australia, the US and now Scotland – all seem to enjoy his training sessions, but he is still an incomer.
Where he is weak is in background knowledge. His role so far had been to have an input when it came to selection but somebody else called the shots. Now he has to look at all the options. He has some impression of those he worked with over the summer and autumn but what does he know about the likes of Johnnie Beattie and Allister Hogg, who were not involved?
He has been given eight matches in charge, which looks more like a trial run than just being thrown in to give the management a chance to get their act together, but when you look at the track record of coaches and managers on similar short-term appointments across all sports, they do not impress. If, for example, Warren Gatland were to leave Wales, would anybody see Rob Howley as the front runner to take over after seven consecutive defeats? Probably not, though he too has a chance to make his mark in the Six Nations.
On the whole, interim appointments do not usually land the long-term job either because their bosses really did have somebody else lined up or because the nature of the role means it is set up for failure; Lancaster was the exception, not the rule. Johnson may join him, but the statistics say the reverse.