Matt Marshall over at SiliconBeat makes a point that is definitely worth making — and one that apparently has to be made over and over again before people get it — which is that Web analytics is (to put it mildly) an inexact science. In fact, looking at the Web-traffic numbers reported by Hitwise, Alexa, Nielsen and Comscore makes the weather-forecasting business look precise and infallible. This is an issue that has come up in the past with MySpace and its growth (as I discussed here) and has now come up again with respect to del.icio.us.

The dancing around in Marshall Kirkpatrick’s recent post at TechCrunch is almost comical, although to be fair at least Marshall is trying to get the story straight. He notes that Mike Arrington wrote about del.icio.us awhile back and was critical because its traffic was stagnating, but then had a chat with creator Josh Schachter and some Yahoo folks (I’m sure no bright lights or sleep deprivation was involved — Yahoo is much more subtle) and now TechCrunch is convinced by a Hitwise report that traffic has doubled.

Stagnating, doubling — tomato, tomahto, right? To his credit, Marshall goes out of his way to note that while Hitwise is a “respected” traffic analysis firm, numbers are all over the map — and he links to the other Marshall’s critique of the field. The simple fact is that Hitwise, Comscore, Nielsen and Alexa all use different methodologies (a good description here) and as a result they are not just talking about apples and oranges, they are talking about apples and oranges and plums and peaches.

When you’re trying to make apple sauce, that’s kind of a problem — and unfortunately all it means is that websites can use whatever data they want to tell whatever story they want, and various blogs and media will lap it up.

I don’t want to add to the “echo chamber” that some have complained about in tech-blogging circles — which is a real risk given the number of blogs tech.memeorandum.com has commenting on the news — but I think it’s interesting that Amazon seems to have decided to open up its Alexa API for no apparent reason.

In other words, there doesn’t seem to have been any pressure to do so, nor is Amazon.com in financial trouble or under severe competitive threat — although it’s true that the company is no longer growing as quickly as it used to. That means it has decided that “opening the kimono,” as Fred Wilson likes to call it, is worth doing for some other reason (Fred calls Alexa “Amazon’s hidden jewel.”)

In all likelihood, it’s because Amazon has seen the spread of Google’s search, not to mention Google Maps, and Google Earth, and Flickr and so on, and realized that an open API likely creates more value — in the longer term — than a closed one. Let’s hope so. Because if there is one lesson that companies can learn from “Web 2.0,” it is that. Paul Kedrosky wonders why the Amazon announcement is news, and maybe it isn’t really. But it is still important.

Update:

Richard MacManus at Read/Write Web has more. And Cynthia Brumfield of IPDemocracy makes an important point (which others have made as well), which is that it isn’t just the open API, but the quality of the index that counts. And Danny Sullivan of SearchEngineWatch is underwhelmed by the news.