Considering the virtual reams of data we generate for companies like Facebook every day, they give us awful little in return. While they sell the information to third parties or use it to display targeted advertisements, we’re left with a largely anecdotal understanding of Internet habits. We can install programs to track our personal Internet usage, but it’s difficult to place these individual habits in a broader context. I may spend two hours a day surfing around, but how does that compare to my peers? Enter Firefox, the open-source project that happens to be the world’s second-most-popular browser. *

Since mid-2009, the folks behind Firefox have encouraged its users to install Test Pilot, a plug-in that collects anonymized browser usage. Test Pilot tracks many different “events” like booting up or shutting down the browser, adding a bookmark, and turning Firefox’s private-browsing feature on and off. (Unsurprisingly, the private-browsing data have received the bulk of the attention.) Every few months, Mozilla Labs, the group in charge of Test Pilot, releases another set of data collected by the plug-in, often examining a specific aspect of browsing—say, what parts of the toolbar people click most. Last month, Mozilla Labs released its most comprehensive Test Pilot data set, the second version of what it’s calling “A Week in the Life of a Browser.”

The abundance of data in “Week in the Life,” which covers a week’s worth of 27,000 users’ browsing activities, can be paralyzing. Faced with several gigabytes of decompressed data, where do you start? Tab usage, I decided.

In the past decade, the ability to open multiple—even dozens of—Web pages in a single window has shifted from a fringe, power-user feature to a mainstream offering. Today, every major browser supports tabbed browsing, even slow-to-evolve Internet Explorer. A browser with 27 open tabs has, fairly or not, come to symbolize the frenetic, attention-deficient aspects of our Web-centric lives. What can Firefox’s data on this feature tell us about ourselves? You’ll find some preliminary answers (and charts!) below. But first a few caveats about the data.

Companies like Nielsen pay people to let software spy on their Internet activity. These companies sell this information to advertising companies, academic researchers, and the like. It has led to handsome profits and fascinating insights. Researchers at the University of Chicago, for example, used Nielsen data to estimate the extent to which Web surfers self-segregate by ideology—a study Slate then used to create a “media isolation” profiler. But if you want access to most of Nielsen’s data, you’ll have to cough up some serious change.

Mozilla Labs, on the other hand, gives Test Pilot data away for free—with tradeoffs. Data collection requires a balancing act between creepiness and the desire for useful detail. Because Mozilla wants to recruit as many participants as possible and isn’t paying them, Test Pilot collects much less personal information than Nielsen does. There’s no information on income, race, or location. In fact, only about 4,000 of the 27,000 users in the latest data dump answered such basic questions as: “What is your gender?,” “How old are you?,” and “How much time do you spend on the Web each day?”

The demographic data Test Pilot users did provide, however, raise a giant red flag. Of course, Firefox users skew toward the type of people savvy enough to use a browser other than the one that comes pre-installed on their computer, typically Internet Explorer or Safari. And though Test Pilot recently passed the million-active-user mark, people who install it represent an even more specific class of technophilic users who are comfortable with plug-ins. (That’s not to say bias isn’t present in the Nielsen panel: It overrepresents people who don’t mind a large corporation tracking their online habits.) What’s remarkable, however, is how enormously Test Pilot underrepresents women—just 6.5 percent of the “Week in the Life” survey respondents said they were female. There are so few female-identifying participants—just 257—that we can have very little confidence whether the women in this study are representative of most other women.

These caveats in mind, let’s look at the tab data. Test Pilot recorded the number of tabs users had open every 15 minutes and every time they opened a new tab or window. In “A Week in the Life of a Browser,” the 27,000 participants quietly logged more than 12 million of these tab-related events. (Two earlier Test Pilot studies also logged tab data, but for fewer participants and in the absence of demographic details.) Let’s start with the most obvious question: How many tabs do users have open, on average? About one-half of users kept an average of more than 2.38 tabs open, and one-quarter kept an average of at least 3.59 tabs open. Thanks to some particularly prolific tabbers (for reasons unfathomable, a participant once had 1,103 tabs open), the average of these averages is 3.2 tabs. Here’s a visual overview, which shows the proportion of participants who kept an average of zero to one tabs open, one to two tabs open, and so on.

What about the extremes of our browsing habits? One-half of the participants maxed out at fewer than eight tabs, but one-quarter of users had 11 or more tabs open at least once during the study. Here’s another histogram showing the distribution of maximum tabs open:

These overall data give us a sense of how we compare to other Firefox users. But what can they tell us about broad demographic trends? As noted, any comparison of the sexes must be done cautiously given how few women participated in the study. That said, men kept more tabs open than women—an average of 4.45 tabs versus 3.86 tabs. Men also kept more tabs open at the heights of their browsing, maxing out at an average of 11.33 to women’s 10.38.

We can also observe how tab usage rises and falls with age:

If you break these averages out by sex, you get this:

Men’s and women’s browsing habits appear to follow remarkably different trajectories with age—that is, if (a very big if) the Test Pilot data correspond to browser habits in the general population. In this sample, at least, women younger than 18 used more tabs on average than any other age-gender combination and far more than men of that age. But while men’s tabbing habits increased after high school, they decreased among women. The odd, mirror-image movements continue through the 46-to-55-year-old groups. Why? If you have a hypothesis, leave them in the comments below.

I’m hesitant to make conjectures about this early data but, if pressed, will offer a few hypotheses. The general trend in open tabs by age group may reflect a balance between increasing responsibility and decreasing Web-nativeness. Juggling life and work may demand more tabs than juggling life and college, which in turn may require more tabs than juggling life and high school (if such a distinction exists). But after a certain age—one’s mid-30s, it seems—the wild Web may be bewildering enough without also having 12 tabs open. The gender split is more perplexing. Perhaps it reflects social and economic structures—like the different career trajectories generally available to men and women. Or, more controversially, perhaps it reflects psychobiological differences between the sexes.

These data alone won’t offer us deeper insight into, say, our distractibility in front of a screen. Before that can happen, we’ll need more data. Though Mozilla continues to release additional Test Pilot results, more robust analyses beg for data from the other major browsers. (Google and Microsoft have collected similar user information for Chrome and Internet Explorer, but they don’t have immediate plans to release the raw data publicly, their press officers tell me.) We’ll also need professional statisticians—which I am decidedly not—to crunch the numbers with greater sophistication. And to make the data relevant outside the boundaries of your computer screen, we’ll need to link it to research in sociology, psychology, and other disciplines. The Danah Boyds of this world could have a field day.

But that’s no reason to wait. The more that academics and laypeople play with these data today, the faster we’ll get to the point that they’re useful. And the analyses above are just a few of the near-infinite possibilities. If there’s an aspect of browsing behavior you’d like Slate to investigate, or if you’d like to share analyses you’ve already performed, drop us a line in the comments.

(To find what sorts of numbers are available for crunching, see the tables on this page.)

Correction, Dec. 6, 2010: This article originally stated that Firefox was the world’s most popular browser. Internet Explorer commands a larger usage share. (Return to the corrected sentence.) Also, originally the article’s first graph, labeled “User Distribution: Average Tabs Open,” mistakenly showed the data set for a distribution of maximum tabs open.

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