The latest landmark for the Dow Jones Industrial Average, which passed 10,000 on Wednesday, should be taken seriously for one reason and one reason only: other people are taking it seriously.

The Dow’s methodological flaws are easy to spot. It is weighted by share price rather than market value, so that one company, IBM, accounts for more than 9 per cent of the index. Meanwhile 3M is more than 5 per cent. ExxonMobil, six times bigger than 3M, has a slightly lower index weight.