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Readme for analog3.90beta1

How the web works

This section is about what happens when somebody connects to your web site, and what statistics you can and can't calculate. There is a lot of confusion about this. It's not helped by statistics programs which claim to calculate things which cannot really be calculated, only estimated. The simple fact is that certain data which we would like to know and which we expect to know are simple not available. And the estimates used by other programs are not just a bit off, but can be very, very wrong. For example (you'll see why below), if your home page has 10 graphics on, and an AOL user visits it, most programs will count that as 11 different visitors!

This section is fairly long, but it's worth reading carefully. If you understand the basics of how the web works, you will understand what your web statistics are really telling you.

I should say that this section has benefited from several earlier expositions of these ideas. In particular, I can recommend four excellent articles: Interpreting WWW Statistics by Doug Linder; Making Sense of Web Usage Statistics by Dana Noonan; Getting Real about Usage Statistics by Tim Stehle; and, the most negative of all, Why Web Usage Statistics are (Worse Than) Meaningless by Jeff Goldberg.

1. The basic model. Let's suppose I visit your web site. I follow a link from somewhere else to your front page, read some pages, and then follow one of your links out of your site.

So, what do you know about it? First, I make one request for your front page. You know the date and time of the request and which page I asked for (of course), and the internet address of my computer (my host). I also usually tell you which page referred me to your site, and the make and model of my browser. I do not tell you my user name or my e-mail address.

Next, I look at the page (or rather my browser does) to see if it's got any graphics on it. If so, and if I've got image loading turned on in my browser, I make a separate connection to retrieve each of these graphics. I never log into your site: I just make a sequence of requests, one for each new file I want to download. The referring page for each of these graphics is your front page. Maybe there are 10 graphics on your front page. Then so far I've made 11 requests to your server.

After that, I go and visit some of your other pages, making a new request for each page and graphic that I want. Finally, I follow a link out of your site. You never know about that at all. I just connect to the next site without telling you.

2. Caches. It's not always quite as simple as that. One major problem is cacheing. There are two major types of cacheing. First, my browser automatically caches files when I download them. This means that if I visit them again, the next day say, I don't need to download the whole page again. Depending on the settings on my browser, I might check with you that the page hasn't changed: in that case, you do know about it, and analog will count it as a new request for the page. But I might set my browser not to check with you: then I will read the page again without you ever knowing about it.

The other sort of cache is on a larger scale. I'm in the UK. Because the link across the Atlantic is sometimes very congested, we've set up a national cache. (Many individual ISP's also do the same thing.) I can set my browser to get your pages from the national cache instead of directly from you. If anyone else in the country has used the cache to look at your pages recently, the cache will have saved them, and will give them out to me without ever telling you about it. So hundreds of people could read your pages, even though you'd only sent it out once. Also, if the page I wanted wasn't already stored in the cache, the cache would ask for it from you on my behalf. This would mean that the request appeared to come from the cache, rather than from me. If several people did this, you would think that only one host was accessing the cache, rather than lots of different ones.

3. What you can know. The only things you can know for certain are the number of requests made to your server, when they were made, which files were asked for, and which host asked you for them.

You can also know what people told you their browsers were, and what the referring pages were. You should be aware, though, that many browsers lie deliberately about what sort of browser they are, or even let users configure the browser name. Also, a few browsers send incorrect referrers, telling you the last page that the user was on even if they weren't referred by that page.

4. What you can't know.
  1. You can't tell the identity of your readers. Unless you explicitly require users to provide a password, you don't know who connected or what their e-mail addresses are.
  2. You can't tell how many visitors you've had. You can guess by looking at the number of distinct hosts that have requested things from you. But this is not always a good estimate for three reasons. First, if users get your pages from a local cache server, you will never know about it. Secondly, sometimes many users appear to connect from the same host: either users from the same company or ISP, or users using the same cache server. Finally, sometimes one user appears to connect from many different hosts. AOL now allocates users a different hostname for every request. So if your home page has 10 graphics on, and an AOL user visits it, most programs will count that as 11 different visitors!
  3. You can't tell how many visits you've had. Many programs, under pressure from advertisers' organisations, define a "visit" (or "session") as a sequence of requests from the same host until there is a half-hour gap. This is an unsound method for several reasons. First, it assumes that each host corresponds to a separate person and vice versa. This is simply not true in the real world, as discussed in the last paragraph. Secondly, it assumes that there is never a half-hour gap in a genuine visit. This is also untrue. I quite often follow a link out of a site, then step back in my browser and continue with the first site from where I left off. Should it really matter whether I do this 29 or 31 minutes later? Finally, to make the computation tractable, such programs also need to assume that your logfile is in chronological order: it isn't always, and analog will produce the same results however you jumble the lines up.
  4. Cookies don't solve these problems. Some sites try to count their visitors by using cookies. But this can only work if you refuse to let people read your pages who can't or won't take a cookie. And you still have to assume that your visitors will use the same cookie for their next request.
  5. You can't follow a person's path through your site. Even if you assume that each person corresponds one-to-one to a host, you don't know their path through your site. It's very common for people to go back to pages they've downloaded before. You never know about these subsequent visits to that page, because their browser has cached them. So you can't track their path through your site accurately.
  6. You often can't tell where they entered your site, or where they found out about you from. If they are using a cache server, they will often be able to retrieve your home page from their cache, but not all of the subsequent pages they want to read. Then the first page you know about them requesting will be one in the middle of their true visit.
  7. You can't tell how they left your site, or where they went next. They never tell you about their connection to another site, so there's no way for you to know about it.
  8. You can't tell how long people spent reading each page. Once again, you can't tell which pages they are reading between successive requests for pages. They might be reading some pages they downloaded earlier. They might have followed a link out of your site, and they might or might not return later. They might have interrupted their reading for a quick game of Minesweeper. You just don't know.
The bottom line is that HTTP is a stateless protocol. That means that people don't log in and retrieve several documents: they make a separate connection for each file they want. And a lot of the time they don't even behave as if they were logged into one site. That's why analog reports requests, i.e. what is going on at your server, which you know, rather than guessing what the users are doing.

I've presented a somewhat negative view here, emphasising what you can't find out. Web statistics are still informative: it's just important not to slip from "this page has received 30,000 requests" to "30,000 people have read this page." In some sense these problems are not really new to the web -- they are present just as much in print media too. For example, you only know how many magazines you've sold, not how many people have read them. In print media we have learnt to live with these issues, using the data which are available, and it would be better if we did on the web too, rather than making up spurious numbers.

Stephen Turner
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