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What is relevance?

Relevance and Recall are the two measurements of search retrieval effectiveness -- in other words, whether you're getting the information you need.

Relevance, also called Precision, means, “Out of all the stuff I got, how much of it was what I really wanted?”

Or, to put it more technically,

Relevance =
Number of relevant items retrieved
[divided by the] Total number of items retrieved

So a search with 60% relevance retrieved 60% good stuff but also 40% junk.

Recall is the other point of view: “Do I think I got enough of the good stuff that’s probably out there?”

Or, to put it technically,

Recall =
Number of relevant items retrieved
[divided by the] Total number of relevant items available

So a search with 60% recall missed 40% of the good stuff.

The ideal search results would have high relevance and high recall -- getting all the good stuff that's out there and not wading through any junk. And there are advanced search techniques you can use to improve your precision and recall. However, in the real world, relevance and recall are usually inversely related: the more you try to improve one, the worse the other gets.

Moreover, in the real world you can't really determine recall: you'll never know for sure how much is out there, and you don't have the time to find out, because the information universe is expanding exponentially. You have to mine your results for tantalizing leads and sniff out their scent, until you can make an educated guess about whether your research is reaching the point of diminishing returns.

You may remember in the early days of the Web, the search engines advertised that they indexed more pages than their competitors. They were selling recall. As Inspector Clouseau said, "Nyot anymeur..." These days, the search engines that couldn't rank the likeliest results at the top are history. What search engines are selling now is relevance.

Relevance is in the mind of the beholder. You need enough good stuff for your needs -- not for anyone else's needs. Maybe that's a hundred evocative images, a literature review of 30 articles, three truly authoritative websites, two specific pages in a book, or the one phone number that will open doors for you. And probably, answering your first question will bring up further questions, and your information needs will change. You'll want to refine your search or try a new direction.

The research process is not a pill for an ache; it's an enlightening dialogue with information. Same goes for the process of envisioning your users' information needs when designing a taxonomy or usability study, or writing technical instructions or a back-of-the-book index.

With twenty years of experience, Relevant Information Services is dedicated to providing you with the highest quality of customer service, by

listening to your information needs,

collaborating with you during the process,

performing expert research or design, and

clearly communicating the results

to bring you or your users the information that is most relevant to you.