There are a few common causes for displaying content differently to different visitors, including search engines. Here are some of the most common ones:
Multivariate and A/B split testing :
Content requiring registration and First Click Free:
In this scenario, you might also opt to participate in a specific program from Google called First Click Free, wherein websites can expose “premium” or login-restricted content to Google’s spiders, as long as users who click from the engine’s results are given the ability to view that first article for free. Many prominent web publishers employ this tactic, including the popular site, Experts-Exchange.com (http://www.experts-exchange.com/ ).
To be specific, to implement First Click Free, the publisher must grant Googlebot (and presumably the other search engine spiders) access to all the content they want indexed, even if users normally have to log in to see the content. The user who visits the site will still need to log in, but the search engine spider will not have to do so. This will lead to the content showing up in the search engine results when applicable. However, if a user clicks on that search result, you must permit him to view the entire article (all pages of a given article if it is a multiple-page article). Once the user clicks to look at another article on your site, you can still require him to log in. Publishers can also limit the number of free accesses a user gets using this technique to five articles per day.
Navigation unspider-able by search engines:
If a significant portion of a page’s content is duplicated, you might consider restricting spider access to it by placing it in an iframe that’s restricted by robots.txt. This ensures that you can show the engines the unique portion of your pages, while protecting against duplicate content problems. We will discuss this in more detail in the next section.
Different content for different users:
At times you might target content uniquely to users from different geographies (such as different product offerings that are more popular in their area), with different screen resolutions (to make the content fit their screen size better), or who entered your site from different navigation points. In these instances, it is best to have a “default” version of content that’s shown to users who don’t exhibit these traits to show to search engines as well.
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