If you were examining Hulu.com’s online audience figures from the last few months, you would find some fluctuation: 43.5 million viewers in May and then 24 million viewers in June. What happened? Did something radically change with the website? Are people abandoning the practice of watching television online?

No, the main change is that ComScore changed its methodology for measuring who used the website. According to the Los Angeles Times:

The three dominant measurement firms — ComScore, Nielsen and Quantcast — have been working since 2007 with an independent media auditing group to make improvements so the Web data they report don’t have a fun-house quality, in which the same site’s traffic can look emaciated or bulging, depending on the viewer’s angle.

These firms have used different measurements over time including panels of users (like Nielsen uses for television and radio) and embedded tags in videos and websites to track viewership. These numbers matter more than ever for advertisers as they will spend around $25 billion in online advertising in the United States in 2010.

As in many cases, knowing the means of measurement matters tremendously for interpreting statistics.