December 8, 2003

On or about November 17, 2003, countless English-language ecommerce sites no longer appeared near the top of the rankings when their owners used the search terms that they considered most important.

Four days later, I discovered that by adding a nonsense exclusion term, the links returned by Google shifted dramatically, and the results were very close to what these site owners had come to expect over the last few months.

A filter was in place, and it could be defeated by using one, or sometimes two, exclusion terms. If an exclusion term consists of characters that would never be found on a web page, then normally the addition of this excluded term to your usual terms will make little or no difference. Under normal circumstances, a search for callback service should return the same links as a search for callback service -qwzxwq because no sane web page has the term qwzxwq on the page.

There have been thousands of posts about the November update on various forums where webmasters trade information. When the exclusion trick was discovered on the evening of November 21, I assumed that we had only the two-day weekend to discover more about this filter. A similar trick using a hyphen between two keywords worked briefly for about three days prior to November 21, at which point it stopped working. It was this previous trick, discovered by someone else, that made me aware of the filter in the first place.

When it stopped working I already knew what to look for, and was in a position to try other tricks. That's how I stumbled onto the exclusion word as a means to turn off the filter.

I announced the new trick on a popular forum and other webmasters confirmed that it worked on their keywords. A day later some webmasters reported that if you use three words in your favorite search term, then you often need two different exclusion terms after them to defeat the filter.

By Wednesday of the following week, it was still working. I didn't expect this, and started this Scroogle site using a script that compared the top 100 results from Google with exclusion terms, against the top 100 results for the same terms, but without the exclusion. I began recording the terms entered by visitors to the site, along with the "casualty rate" for those terms. This rate is the number of links in the unfiltered top 100 that were missing from the filtered top 100.

This so-called "Hit List" is a moving window of the most recent 10,000 terms entered by visitors, minus duplicates and some porn. (You should be aware that porn terms are underrepresented on my Hit List of filtered terms, because I delete the most offensive ones.) There is also a cutoff on the low end to keep the file size reasonable. This means that you may not see many terms with casualty rates under five or so. The Hit List is only a sampling of the terms that visitors to this site have entered, mostly in the last 24 hours. It's not definitive, and all it can do is give you an idea of which terms get filtered.

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