Relevancy ranking in Summon

Yesterday, Tim Fletcher tweeted me a question about Summon:

How does Summon rank results? is there a logic?

…it’s not the kind of question that you can answer in 140 characters, but I quickly knocked off an email to Tim. This morning David F. Flanders suggested I should also blog the response.

So, first of all, a quick caveat: much of the following was gleaned from various presentations over the last couple of years or so and may not be 100% accurate (I’m particularly good at misunremembering stuff!)

The first time I saw Summon (back in early 2009), I believe Serials Solutions were still using the default relevancy ranking that comes with the Open Source Lucene software (which is documented here). In a nutshell, Lucene generates a score for each indexed item (that matches the search query) and then those items are sorted by score (in descending order) to produce the ranked results.

I’ve read quite a few times that the relevancy ranking engine in Lucene is regarded as one of the best, which might be one of the reasons why SirsiDynix recently moved Enterprise from using Brainware to Lucene.

When you mention Lucene, chances are Solr won’t be too far behind. Solr (which is also Open Source) extends Lucene to provide a host of extra features, including facets.

As Summon has developed, and in response to customer feedback, Serials Solutions have gradually tweaked the way their Lucene installation generates the scores by giving each result an additional boost (or reduction) depending on a variety of factors, including:

  • Currency – newer items are given a slight boost over older items
  • Content type – books, ebooks and journal articles get a boost to their scores, whilst newspaper articles and book reviews have their scores reduced
  • Local collections – things that come from the user’s library (e.g. books, repository items, local archives, etc) get a little boost

Additionally, the Summon search engine handles certain words and phrases differently. For example, Lucene normally treats the singular and plural version of words as the same, so searches for “africa hospital” and “africas hospitals” both bring back roughly the same number of results. However, Summon understands that “africa aid” isn’t the same thing as “africa aids“.

Given that few users go beyond the first page of results (I was told the exact figure last week, but it’s slipped from my memory — I think it was less than 5%?), Serials Solutions put a lot of effort into trying to ensure that the most relevant results appear on that first page. Given that the Summon master index is fast approaching 1,000,000,000 items, that’s no trivial task!

As they say, the proof of the pudding is in the eating, so feel free to run some searches on our Summon instance to see how well you think it ranks the results.

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