How many readers does Slate have? Like other Web sites, we get asked this question all the time. Our usual reply is: "Thank you for asking. That's a complicated question." But here is our best effort to answer it, along with an explanation of why counting readers on the Web is so complicated (and why you should take any claims about Web-site traffic--except ours, of course--with a grain of salt).
Traditional print magazines know exactly how many copies they sell, though they have no idea how many people actually read a particular article or see a particular ad. TV networks rely on sampling by companies like Nielsen to estimate how many viewers were watching which channel at any hour of the day. (See Slate's recent "Dispatch" from someone selected to be a "Nielsen family.") Nielsen and other firms such as PC Meter, now known as Media Metrix, are scrambling to adapt this concept to the Web. Meanwhile, though, there are hits.
The contents of a Web site are stored on high-performance computers called servers. When you type a Web address into your browser, or click on a link or a "favorites" or "bookmark" button, you are telling your computer to fetch a specific set of data--text, images, sound, etc.--from some Web site's server. Your browser then assembles these data and displays them as a page. Each of these requests for information is a hit.
Hits (per day, per week, per month ...) are the most common measure of Web traffic. But they are deceptive. A single Web page can be one hit or many, depending on how it is constructed. The more separate elements it contains (images, sound ...), the more hits it will require. That's why Slate, and some other sites, prefer to talk about pages served. This is a measure of how many complete pages your server has sent out in response to requests from browsers. In recent months, Slate has been serving an average of about 90,000 pages a day.
But the number of pages served is also a misleading measure in some ways. A "page" is not a standard unit on the Web the way it is in print. Depending on a Web site's design, the same amount of content can take up a very different number of pages. Slate is designed so that every article or feature takes up a single page. Other sites break up articles into many pages. (We're not suggesting that people are padding their page counts. It's a judgment call about whether readers will find having to scroll more annoying than having to click and wait.)
Also, sometimes a site will feed you two pages when you've only asked for one. Slate does this, for example, with our cover and contents pages. However--Boy and Girl Scouts that we are--we subtract this double counting in the figures we report. We cannot guarantee that every site is so scrupulous.
Further complicating page counts is the issue of caching. Computers at corporations and other large organizations often access the Web through a middleman computer known as a proxy server. Your computer requests a page from the proxy server, which then requests it from the Web site. But proxy servers are often programmed to save, or cache, frequently requested pages, rather than retrieving them from the Web each time they are requested. America Online also caches pages for its customers. And your own computer is probably set up to cache some pages you've visited recently. (That's why, when reading Slate, it's quicker to go back to the contents page than it was to call it up at the start.) All this caching means pages are served more quickly, but it does become harder for a Web site to know how often its pages have actually appeared on someone's screen.
Anyway, 100 pages served could be 100 people reading this column, or one obsessive participant in "The Fray." What we really want to know is how many individual readers Slate has. And we can find out, sort of. Each request to Slate from your browser carries with it an assortment of useful information. Your computer tells us what operating system and browser software you are using, so that our server can return a page appropriate to your setup. We also are told about the referring page--that is, the page you were reading when you requested this one. (Marketers love that information.) More important, each page request is accompanied by a return address. How else would the server know where to send the data? Every computer connected to the Internet has an address, known as an IP address, consisting of four numbers. For example, 220.127.116.11 is currently the IP address of one of Slate's servers. Depending on how you access the Internet, your computer has either a permanent or a temporary IP address.
But for a variety of reasons, including caching, IP addresses aren't a great way of counting individual visitors. Therefore, there are cookies. Cookies are like the club bouncer who stamps your hand when you leave the premises temporarily, so as to identify you when you return. The first time you visit Slate, our computer sends yours a piece of data, which your computer sends back every time you return, so we know it's you. Cookies aren't perfect. They don't account for multiple people accessing the site from one computer, or one person using different computers (or browser software). Also, some older browsers don't understand cookies, and newer ones allow privacy freaks to turn them off. But, using cookies, we can get a pretty good idea of how many unique browsers are visiting our site.
Even that, though, is a problematic concept. Do you measure browsers per day? Per week? Per month? The longer the time period you choose, the more individual visitors you can claim, which is nice. On the other hand, because of repeat visitors, the number of unique browsers in a given month is less than the total of unique browsers per week for those four weeks, which is less than the total of unique browsers per day over those 30 days. Choose your poison: Slate attracts about 6,000 unique browsers a day and 80,000 a month.