This is a season that tries the spirit. It's not just the inescapability of seasonal affective disorder, though how anyone in latitudes north of Watford survives the lightlessness of January and February is a mystery. The particular gloom of January for an admissions tutor such as myself is the misery that comes from trying to explain to unhappy schools, unhappy parents and unhappy candidates why they haven't been given a place at Oxford University. Everyone in the same position elsewhere will suffer in the same way later.

Some unhappiness is self-inflicted, or perhaps inflicted by the "blame culture" we are said to inhabit. The first question is always: "What did I (or he or she) do wrong?" Most of the time, the answer is: "Nothing."

Someone who applies to study a vastly oversubscribed subject such as economics and management, where a dozen well-qualified applicants are competing for each place, is much like a marathon runner who runs the race of her life, beats all her personal best times and comes in third behind two runners who squeeze out a slightly better performance on the day. A student can do absolutely nothing wrong and still be turned down.

Trying to get that point across is not easy. It produces the reaction: "But if they are that good, why didn't you take them?" And the answer is painful both to give and to receive; when terrifically good people are competing with each other, some terrifically good people lose out. The fourth-fastest sprinter in the world is no slowcoach but won't be standing on the podium.

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Schools don't make life easier by - quite reasonably - doing all they can to encourage their pupils. Overestimating their chances may encourage them to put in the work and to be sufficiently ambitious, in much the same way that most schools over-mark students' work in the run-up to GCSEs to provide encouragement. But it makes disappointment harder and also less explicable; that is no kindness. Teachers can hardly help succumbing to wishful thinking - I have mistaken geese for swans at every level from entrance candidates to candidates for named chairs at grand univer-sities - but then they are not just disappointed but puzzled. Puzzlement fuels the belief that we have been unreasonable; and that fuels outrage.

It is, of course, impossible to be confident that the selection process gets it right. Since the occasional student turns out to be dim, idle, wicked or all of these, it is clear that mistakes are made. My doubts go deeper. Everyone tries very hard, operates honestly, chooses without fear or favour; processually speaking, virtue is the order of the day. But I'm not convinced that we know how to do what we are trying to do. We look for "academic merit", but that is not a simple notion. It's more about promise than existing achievement; past success is a clue, but only a clue - we need to know what applicants will do next. But promise is notably hard to measure. Ten per cent of all who apply will do excellent work anywhere; of the rest, some have peaked, some are late developers, some are naturally self-propelled and some will always need a teacher's help. We are trying to make predictions on very little evidence.

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But when you are on the receiving end of unhappy letters you frequently have to explain why someone looked only very marginally less promising than the competition. And as we learnt in Sociology 101, the worst place to make fine distinctions is at the top of the bell curve. If you are making fine distinctions with fuzzy concepts such as "promise", you can't help having qualms. It's not that you think different decisions would have been better; if you did, you'd have made them. It's more that, on the evidence available, you just can't be sure you have got it right.

Which is why at this time of year it always occurs to me that we would be better off behaving like the Dutch and the Irish and allocating some - I'd say a third - of our places by lottery. When critics say that Oxbridge admissions - really, admissions in any of the dozen most competitive universities - are a lottery, they mean it as a complaint; but if it is true that a third of the students admitted could without injustice have been selected at random from a pool from which the shoo-ins at one end and the no-hopers at the other had been eliminated, a lottery would be quicker and fairer. And speaking person-ally, I'd much rather that the unsuccessful candidates, along with their schools and their parents, spent January yelling at the computer than complaining to me about my colleagues'

judgment.

Alan Ryan is warden of New College, Oxford University.