The former US Federal Reserve chairman Paul Volcker once wrote: "The fate of the world economy is now totally dependent on the growth of the US economy, which is dependent on the stock market, whose growth is dependent on about 50 stocks, half of which have never reported any earnings."

That truism was put in focus this week in the US markets' topsy-turvy performance. Markets have been sold off their highs in recent months by a concern about the valuation of some stocks, especially those in the telecoms and technology areas. More recently investor worry about the likely shape of third-quarter corporate earnings saw the indices slip back further. So nothing that happened this week was a surprise. Companies were merely fulfilling the lowered expectations of the markets.

But the investor response to the production of figures has been marked. When groups like Intel, Apple and, particularly, IBM reported disappointing sales, the markets took a shower, especially the technology-heavy Nasdaq.

Then Microsoft and Nokia came in with bullish figures and sentiment turned on its head. Granted, the two are leaders in their sectors but if investors were right in the first place about the outlook for the market, two good sets of figures hardly justify an 8 per cent market surge.

The vagaries of the US markets would not be so frustrating had they less of an influence on other exchanges worldwide. Investors trying to gauge the future mood of the markets at this point might as well stick a pin in the index of their choice.