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Suppose you have a number of competing candidates, each of which can be ranked with a score, but it takes a little time to calculate each candidate’s score. You’re only interested in the top candidates. You want to come up with a scoring scheme where you can throw the extra candidates out of consideration earlier without sacrificing quality. Such is the problem of scoring and ranking suggestions in Ubiquity. What properties must such a scoring system have?

This blog post includes a lot of complex CSS-formatted graphs which may be best viewed in — what else? — Firefox. You may also want to access this blog post directly rather than through a planet.

One portion of the problem description above merits clarification: I define “without sacrificing quality” to mean that, if we did not throw out any candidates early and waited until all the scores are computed fully and accurately, we would still yield the same top winners. This already gives us the key insight towards an appropriate solution: we can only throw out candidates when we know that it has no further chance of making it up into top candidates.

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I just spent some time reviewing how Ubiquity currently ranks its suggestions in relation to to Parser The Next Generation so I thought I’d put some of these thoughts down in writing.

The issue of ranking Ubiquity suggestions can be restated as predicting an optimal output given a certain input and various conflicting considerations. Ubiquity (1.8, as of this writing) computes four “scores” for each suggestion:

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