Measuring online readers

The growth of online advertising is being stunted, reports the New York Times, because nobody can get the basic visitor counts straight. An excerpt:

Online advertising is expected to generate more than $20 billion in revenue this year, more than double the $9.6 billion it represented as recently as 2004. Nobody doubts that the figure will grow — particularly as advertisers hone their techniques for aiming messages to particular consumers — but the question remains how much the clashing traffic figures will hold the market back.