The proof is in yesterday's announcement of the takeover of American Interpublic - the world's fifth largest advertising group - by its compatriot Omnicom, currently second on the podium ahead of the UK's WPP, but still lagging behind France's Publicis.

The news comes just a few days after a PR campaign launched by Publicis to celebrate - with the help of rapper Snoop Dog and a bit of boasting - its position as world leader. Subject to approval by the competition authorities - which seems sooner said than done - this campaign will now be eclipsed by the Omnicom-Interpublic combination.

Financed entirely in Omnicom shares, the Interpublic takeover confirms what we pointed out in our previous commentary on the group's results, namely that its current valuation was evolving on historically high multiples, which its chairman John Wren apparently wanted to take advantage of.

Valued at $13 billion, Interpublic, whose sales have grown by only 25% over the last decade, is being bought out at thirteen times earnings, a multiple quite comparable to that which the market was assigning to Omnicom, which incidentally has had a comparable growth performance.

Skillfully managed, a formidable cost-cutter and opportunistic buyer of its own shares on the market, Interpublic had nevertheless managed to triple its earnings per share over the period. In this respect, a valuation of just thirteen times consolidated earnings is surprisingly modest.

Combined, the two groups, which will bring together under one roof the well-known agencies McCann, FCB, BBDO and TBWA, among others, will generate an operating profit before depreciation - or EBITDA - of $4 billion. Based on the ten-times EBITDA multiple that has largely defined their valuation over the past decade, the new entity should command a market value in the region of $40 billion.

This transaction has drawn the sarcasm of a few well-informed observers, including the volcanic Martin Sorrell, founder of WPP and S4, who compared it, not without humor, to "a caravan of wagons going into a defensive circle" in the face of the big technology groups that now capture the bulk of advertising business.

It also reopens the debate on the valuation of Publicis - which is evolving at record levels - and on possible further consolidation in the sector. As we know, S4 and WPP are both going through difficult times, while the smaller Havas, soon to be listed on the Dutch stock exchange, is likely to pale in comparison with the sector's behemoths.