How PeerIndex calculated the Twitter 100

The Twitter 100 has been compiled with the help of the social media monitoring group, PeerIndex, with additional input from a panel of experts.

The PeerIndex ratings attached to each entry are calculated by measuring people's Twitter presence in terms of impact, eminency or leadership, on a scale of 1 to 100. A rank of 100 is the maximum possible; a score of 90 or more puts you in the top 0.01 per cent of the population; the average score is 19. The key to these measurements is that they try to measure how other people respond to tweeters. Your rating is a function of other people's assessments and not your own. This makes it harder for people to "game" the ratings.

Broadly speaking, we have measured the nation's tweeters in three major areas:

• Authority: How well does a person resonate with their audience, and with the world at large? How likely are they to say or share things other people will find interesting? PeerIndex calculates authority using an algorithm called "eigenvector centrality" (also used in Google's PageRank technology). Essentially, this calculates the extent to which other people "'vote" for you by retweeting or commenting on your tweets.

• Audience: How many followers does a person have? But, also, how engaged is the audience? Does the tweeter engage in conversations? And do those followers answer back?

• Activity: How active is a person in driving their authority and audience?

Different people on the list earn their spurs in different ways. Some celebrities have many followers but appear to make little impression on them. The PeerIndex ratings allow us to compare unlike with unlike in a reasonably objective way. However, The Twitter 100 is based on more than mere number-crunching: otherwise it would be dominated by celebrities and technology journalists, to the exclusion of some of the most significant tweeters in other fields. Twitter is not yet evenly distributed: some fields, like technology and science, have very large communities of fans; others, like literature or art, have more incipient Twitter communities – but are none the less plainly influential. So we also searched specifically (further down the rankings) for people who had particular resonance in certain fields; and further refined our list by focusing on those who were especially trusted by other experts. This was where the advice of our expert panel came in.

Our five-person panel comprised Azeem Azhar, founder of PeerIndex; Ian Burrell, media editor of The Independent; Julia Hobsbawm, founder of Julia Hobsbawm Consulting and the media networking business Editorial Intelligence; Steve Moore, director of the Big Society Network; and Stefan Stern, cultural commentator and director of strategy at the Edelman PR agency. They analysed the initial results of PeerIndex's calculations, made recommendations and disqualifications, and argued for extra weighting for certain key tweeters whose significance to the medium is, in their expert judgement, greater than the numbers imply. Such interventions were kept to a minimum but give our final list the crucial virtue of having been assembled through the prism of human intelligence.

It may be imperfect, but it is, we believe, the nearest that anyone has yet come to an authoritative who's who of the Twitter world.