Ask HN: algorithm used in Hacker News, that determines the front page news

Gravity.

Score decreases over time, so interesting news bubble up, and older/no-longer-relevant news gradually decline.

    (= gravity* 1.8 timebase* 120 front-threshold* 1
       nourl-factor* .4 lightweight-factor* .17 gag-factor* .1)

    (def frontpage-rank (s (o scorefn realscore) (o gravity gravity*))
      (* (/ (let base (- (scorefn s) 1)
              (if (> base 0) (expt base .8) base))
            (expt (/ (+ (item-age s) timebase*) 60) gravity))
         (if (no (in s!type 'story 'poll))  .8
             (blank s!url)                  nourl-factor*
             (mem 'bury s!keys)             .001
                                            (* (contro-factor s)
                                               (if (mem 'gag s!keys)
                                                    gag-factor*
                                                   (lightweight s)
                                                    lightweight-factor*
                                                   1)))))

direct copy from http://news.ycombinator.com/item?id=4746566

According to the link in dmgrow's comment, it's

    Score = (P-1) / (T+2)^G

where P is the number of votes, T is the time in hours since submission, and G is a power that controls the steepness/rate of decline in ranking, so popularity is decreases exponentially over time. I'm sure that YC-affiliated posts have a different ranking algorithm, considering you can't vote on them.

Some other cool algorithms are:

del.icio.us / delicious.com 's: ranked by upvotes in the last hour. I like the elegance of this solution a lot :)

reddit's "best" feature: compares the rates at which comments are upvoted to generate the probability that a comment is a good one, which is how posts with fewer votes can be ranked better than posts with more votes. It works well, in my opinion.

I think there's more to it than what the other two comments described. No doubt there is human moderation on HN (beyond users flagging submissions I mean), and maybe a list of domains and keywords to keep some stories from ever hitting front page, even when the submission gets a lot of votes in a short amount of time.

Who knows what the reasons why they'd keep a story off the front page. The moderation is emo at times, even resorting to hellbanning, slowbanning etc.

Slashdot moderation is way better. It's a better overall system over there.