Friday, 15 June 2012
Google's PageRank and Alexa Traffic Rank are two very different scores which both try to capture the relative "importance" of sites on the web.
I only discovered Alexa a couple of months ago, whereas I've had to study Google's PageRank algorithm as part of my doctorate research - but Alexa seems to be gaining in popularity, and if you accept advertising on your blog or want to deal with big PR firms, then it's getting more and more likely that your contacts will check out your Alexa rank as well as your PageRank. So I thought it might be worth writing up a quick summary of the similarities and differences between these two systems.
(There are several other ranking systems around, but at the moment, PageRank and Alexa seem to be the big players. Perhaps that's not so surprising, given that they're owned by Google and Amazon respectively.)
PageRank is calculated primarily from links between other sites and yours. This is Google's method of determining the relative importance of web pages: sites with lots of links in are likely to be more valuable. Not only that, but links from sites with higher PageRank are themselves given more weight. As part of Google's algorithm for deciding which search results to display (and in what order), the PageRank score of a site is used along with the relevance of a particular article to a particular query. 10 is the highest possible score, and PR10 sites are very rare, but tend to be things everyone has heard of (like Twitter).
On the other hand, Alexa measures the amount of traffic actually visiting your site, at least in theory. However, it can only track visits from users who have the Alexa toolbar installed, and this can lead to some very skewed results as the toolbar tends to be more popular with certain sections of the population (such as webmasters). The Alexa rank gives your position relative to other sites, and lower numbers are better. For an example of the information Alexa provides for free, see the Alexa page for rachelcotterill.com.
It might seem that both of these things are out of your control, as a site owner, but there are a few things you can do to increase both your PageRank and your Alexa rank. Overall, I would say it's more important to concentrate on your PageRank, as this will directly affect your position on the most popular search engine. Alexa rank won't bring you visitors, but it's worth bearing in mind if you want to sell advertising or get products for review, because it's widely seen as an approximation of your web traffic.
What Constitutes "Importance"?
Google takes a rather theoretical approach to what "important" means online. The original, published PageRank algorithm considers the question "How likely is it that someone, browsing the web and clicking links at random, will come to this page?" Later Google algorithms have started to take into account the relevance of the pages linking in to a site, so if you have a recipe post, then posts from other food pages will be more highly valued; this dilutes the importance of the 'pure' PageRank metric, but the principle is the same, just with a few added metrics. Conversely, Alexa is more focused on what real people are actually doing, in real time. Although this is a limitation (because they can only see a small subset of the data), it's arguable that whether people actually visit is likely to better reflect a site's importance.
Both sites care about inbound links (i.e. links from other sites to yours). Google measures the number of inbound links to your site by spidering the internet, looking at each page to see where it links to. Alexa's measure of inbound links, like its traffic measure, is derived from users who have the toolbar installed - so an inbound link is only counted by Alexa when someone actually follows that link (while using the Alexa bar). Alexa displays the number of inbound links as part of its site information, but this doesn't affect your Alexa Traffic Rank, whereas for PageRank the number (and quality) of inbound links is a central factor in the ranking.
PageRank, as published, is a score from zero (unranked) up to 10, with 10 being the best possible rank, but it seems likely that Google uses more fine-grained scores internally. Although Google continuously updates its internal scoring, public updates to PageRank are only published about every three months, meaning that a new site can stay "unranked" for quite a while. Alexa attempts to put all websites in order, from #1 (the best, which is http://google.com at the time of writing) down into the multi-millions. Updates are approximately daily, so you can watch your score fluctuate, though occasionally it sticks for a couple of days.
There are various browser plug-ins which will let you see the PageRank and Alexa rank of any site you visit. This can be a good way of keeping an eye on your rank, and makes it easy to spot any changes (for better or worse). The Webrank Toolbar (for Firefox, at https://addons.mozilla.org/en-US/firefox/addon/webrank-toolbar-pagerank-alexa/) gives enough stats to keep you occupied for hours! You can check the PageRank of an individual page at http://prchecker.info and Alexa ranks at http://alexa.com.
Want some ideas to improve your Alexa rank?