How do the favorite/ignored tags influence the main questions feed?

Question

If you've just created a new Stack Overflow account, the main feed contains all the available questions: there's no filter applied on it. Then, you can add your Favorite Tags and your Ignored Tags. Since I find the feed is particularly accurate and well filtered, I wonder:

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1) How do the favorite tags influence the feed?

I would have said that when the first favorite tag has been entered, only questions that are linked to that tag get displayed, but it would have been too restrictive. The idea is not "the more you tag the more you see" but rather "the more you tag the less you see". In other words, if there are 100 questions at the beginning:

• what should not be: 1 tag > 10 questions, 2 tags > 20 questions...
• what should be: 1 tag > 90 questions, 2 tags > 80 questions...

Stack Overflow implemented it the right way: there are still displayed questions that aren't tagged with any of my favorite tags, but these questions are (usually) still interesting to me. How do they get chosen? Why these ones and not others? Are there relations between tags (i.e. not only synonyms but also linked tags)? If yes, how are they set? Collaboratively or by an admin (or both)?

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2) How do the ignored tags influence the feed?

It seems to be more understandable, since you could think that questions tagged with any of the tags I ignored won't be ever displayed. But once again, I already saw displayed questions that were tagged with one of my ignored tags. Why? How have they been chosen?

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3) What choice has been made when a question is tagged with a favorite tag AND an ignored tag?

Does the question get displayed or not? It seems also that it depends on criteria, but which ones?

Answer 1

Per this SE blog post, the "Interesting" list is determined by this algorithm:

Starting with a list of the last 3,000 active questions:

• drop questions containing any of your ignored tags
• drop closed questions if you lack the reputation required to vote for reopening
• drop questions scoring -4 or lower

Next, apply the following score formula to the remaining questions:

your interesting tags +1,500 per interesting tag, up to +2,000 total your top 40 scoring tags max of +1,000 per tag (scaled), up to +2,000 total question score +200 × score, up to +1,000 total total answer score -200 × score, up to -1,000 total number of answers -200 × answers, up to -1,000 total number of views -15 × views, up to -1,000 total question last activity date -1 × (seconds / 15)

Count it all up and take the top 90 by score.

We also mix in a few random questions from the last 3,000 — 10% (9) for logged in users and 20% (18) for anonymous users. We’re like DJs trying to spin a mix of songs — some you might know by heart and love, others you might not have chosen for yourself, but could possibly like if you gave them a fair listen.

That should answer your first two questions about how favorite and ignored tags affect the list, and for your 3rd question, I would assume if a post was tagged with both a favorite and ignored tag, it would get dropped in the very first step.

So the reason it's so good at picking questions you'd be interested in is because it considers the tags to your posts, and the community votes or activity on the post in addition to your favorite and ignored tags.

Answer 2

I try to answer this based on my experience, but I don't have any other source.

1. Favorite tags

   • Don't aggregate more question of that type
   • Change only the background of questions with that tag (if you did not search for them).

2. Ignored tags

   • Are removed client-side (with JS)
   • Just don't display.

Questions that have both ignored and favorited tags are ignored (I once ignored ns2 because it appears mostly with my favorite tag tcl, but I don't have the nessecary domain knowledge to answer most of them.)

I have a set of low-volume tags as favorites, so I only see a single question with a sqlite on stackoverflow.com.