5 February 2015 Last updated at 10:48

Who, What, Why: How does Google's Autocomplete censor predictions?

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People in the UK and US were being offered "isis" as one of the top suggestions when typing in the words "how can I join". Google does censor some search suggestions, so why was this one allowed through, asks Justin Parkinson.

As of 0900 GMT on Thursday if you entered "how can I join" into Google in the UK, the search engine, using its Autocomplete predictor, offered several suggestions. First was "the police", second "the illuminati" and third "a union".

The fourth suggestion was "isis".

Also known as Islamic State, IS or Isil, the jihadist organisation, which controls large areas of Iraq and Syria, has reportedly recruited more than 1,000 people from the UK to fight for it.

But Google, alerted to the Autocomplete options by the BBC, had by 1030 GMT removed "isis" as one of the suggestions.

Danny Sullivan, founding editor of the specialist news website Search Engine Land, says Isis was also offered as a choice by Autocomplete when "how can I join" is typed into Google in California. When he tried it, it came second, after "the illuminati" and ahead of Disney's "Club 33" and "kkk" (Ku Klux Klan).

Isis is known as a highly effective user of social media, including Twitter, Facebook and Instagram, with which it spreads its messages and disseminates pictures and videos, encouraging its followers to do the same. The group and its supporters have also reportedly used video games, leaflets and videos to recruit.

Google says Autocomplete's algorithm-based suggestions are based on "a number of factors, including the popularity of search terms". It looks for "200 unique signals or 'clues' that make it possible to guess what you might really be looking for", including one's geographical region. Relevant search terms individuals have used in the past can influence the choices offered.

Google has previously removed certain words from Autocomplete suggestions, including swearing and some sexual language, on grounds of taste or legality.

"We periodically update our systems to improve Search, so the terms that appear in Autocomplete may change over time," a spokesman says. "We exclude only a narrow set of search queries such as those related to pornography, violence, hate speech, and copyright infringement."

Last month, the UK and US pledged to work jointly to prevent the spread of extremist ideologies.

"It may be that people are simply typing it into Google for research purposes, to find out about Isis, rather than simply wanting to join it themselves," says Sullivan. "But the reason we're seeing Isis on Autocomplete is the number of people typing in 'How can I join Isis'."

Any would-be jihadists won't learn much that is useful. The search brings back mostly mainstream media articles about IS and its recruitment methods.

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