The tiresome cliche "size matters" is true in a wide variety of contexts, but the thing measured is at least as important as its magnitude. On the Web, Internet metrics companies have long focused on page views, but many other things may merit measurement.

Nielsen//NetRatings said Tuesday that it has added both "total minutes" and "total sessions" metrics to its NetView syndicated Internet audience measurement service.

The reason is that Web pages that rely on Ajax technology or present streaming media can serve new content without requiring the user to reload the page. And clearly a Web page visitor who spends 20 minutes watching a YouTube video isn't the equivalent of a Web page visitor who spends 10 seconds glancing at a news article, at least as far as advertisers and publishers are concerned.

"'Total Minutes' is the best engagement metric in this initial stage of Web 2.0 development, not only because it ensures fair measurement of Web sites using RIA [rich Internet applications] and streaming media, but also of Web environments that have never been well-served by the page view, such as online gaming and Internet applications," said Scott Ross, director of product marketing for Nielsen NetView, in a statement.

Tracking the time spent by users on a Web site may be an alternate way to measure engagement that tracking page views, but it remains to be seen whether it's better. Perhaps the best that can be said of time tracking is that it too has its uses.

Even so, it's only a matter of time before unscrupulous Web site operators design their Web pages to delay visitors as a way to make their sites appear more engaging. And there are a variety of possible scenarios by which a user might have multiple browsers, browser windows, and browser tabs open while paying attention to none of them.

What's more, search sites like Google.com, which is optimized for speed, aim to minimize the amount of time users spend on-site before they follow a link. Time spent just isn't the best way to measure the success of a search engine.

Using the "total minutes" metric, the Top Five Web brands in May were AOL Media Network (25 billion minutes), Yahoo (19.6 billion), MSN/Windows Live (10.6 billion), Fox Interactive Media (7.8 billion), and Google (7.4 billion), according to Nielsen//NetRatings. Using the current "unique audience" as a metric, Google (110.2 million) comes in first place rather than fifth place.

Google declined to comment, citing a policy of not commenting on third-party market research.

Like the search industry, online metrics companies still has a ways to go in terms of transparency and trustworthiness. A recent report by spyware researcher Ben Edelman noted that comScore doesn't always obtain user consent before installing its tracking software. And a spat in April between the Internet Advertising Bureau, comScore, and Nielsen//NetRatings showed that there's disagreement in the ad industry about how to measure online audiences.

In March, blogger and Edelman senior VP Steve Rubel for auditing online audience data. "Everyone tries hard to spin data a certain way to make it seem like they're delivering more qualified eyeballs/ears than their competitors," he wrote in blog post. "Auditing keeps publishers humble."