Sam Gyimah's proposal that universities be ranked by graduates' earnings has been criticized as missing the point that the purpose of a university education is more than personal enrichment. There are, however, other problems with it. I'll take just six:

First, at what stage of graduates' careers do you measure earnings? If you do so soon after graduation, a course that produces lots of (say) investment bankers will seem better than one that produces lots of (say) barristers, because bankers earn big money quickly whilst barristers take longer to make it. If, however, you measure earnings later in life you risk using out-of-date information. The earnings of 50-somethings might tell us something about what university courses were like in the 1980s, but what does that tell us about them today?

Secondly, there's a massive variation in earnings. Suppose that most graduates from a course earn average money, whilst one makes a fortune. Mean earnings will then be high, but median ones low. Which is the better guide for a student? The more we live in a "winner-take-all" or superstar economy, the bigger this problem will be.

Thirdly, qualifications alone do not determine earnings: a passing glance at your contemporaries will tell you this. As Bowles, Gintis and Osborne wrote in a classic paper (pdf):

Apparently similar individuals receive quite different earnings: a person's age, years of schooling, years of labor market experience, parents' level of schooling, occupation and income tell us surprisingly little about the individual's earnings.

Many other things matter as well: soft skills; agreeableness ("being nice doesn't pay" says (pdf) Guido Heineck); narcissism; psychopathy; looks; and so on.

Maybe differences in all these will cancel out over large numbers. If, however, we are looking at particular courses in particular universities we are not dealing with very large numbers. And if average earnings are skewed by a few high earners, such differences will distort the results.

Fourthly, earnings can be enhanced by irrationality. In particular, overconfident people are likely to do better than ones with a rational assessment of their ability. A good university - surely - would teach its students to be rational. To the extent that it succeeds, though, it might impair its students' earnings relative to a university that inculcates unjustified confidence. To this extent, earnings-based rankings will favour bad universities over good.

Fifthly, and perhaps relatedly, we know that people from posh backgrounds earn more than those from poor ones even with the same qualifications. If we do not correct for this, a university that accepts more posh students will look like a better one than one that accepts fewer.

Sixthly, any university worth the name will encourage a sense of curiosity in its students. This, however, might not enhance their earnings. Lawyers might take on interesting pro bono work rather than stick to the dull day job; investment bankers will get bored and look for more interesting if less remunerative careers; or people might just ditch 60-hour weeks to do give themselves time for other things. Adam Smith wrote about compensating advantages because there really are such things.

Ranking universities by graduate earnings is, therefore, daft. It fails to heed the message of Jerry Muller's new book, The Tyranny of Metrics. "What is most easily measured is rarely what is most important" he says. And, he adds, metrics can be gamed: all the above suggests that if universities are judged by earnings, they'll want posher students; will encourage them to enter careers where financial success comes earlier; and will discourage rationality and curiosity.

When people are stupid, though, we must ask why. I suspect Mr Gyimah's proposal is founded upon an unwillingness to face two uncomfortable facts.

One is that teenagers' decisions to invest in their "human capital" are in many cases taken in conditions of almost perfect ignorance - not only about the future shape of the economy but also about their future tastes and preferences. A desire to rank universities by some objective-sounding measure represents an attempt to create an illusion of knowledge that hides this overwhelming uncertainty.

Also, our earnings are only very partially determined by our own efforts and choices. Luck plays an important role and merit might sometimes even be a handicap. Naturally, though, Tories want to overlook this fact.