Last updated: 11/26/2015, 12:19 PM UTC

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Two Programmers Claim Reddit's Voting Algorithm Is Flawed

Two programmers have separately concluded that the algorithm Reddit uses to sort its posts is flawed in a way that discriminates against new posts that briefly trigger a negative reaction from Reddit readers. That seems trivial at first, especially if you don't understand code. Reddit appears to be working just fine - so who cares about a typo among thousands of lines of code? But Reddit is a massive distributor of traffic around the web. It had 90 million unique users visit its pages last month. Publishers (Business Insider included) benefit hugely when a post becomes popular on Reddit. A single hot link from Reddit can pour hundreds of thousands of readers into your site within hours. And those pageviews are easily monetized with ads. So there is a lot at stake. People trust Reddit to get it right. Ian Greenleaf, a San Diego-based programmer, claims in a blog post that the sorting mechanism Reddit uses to rank new posts can bury those posts if they initially receive a few negative votes. It's complicated, but basically Reddit's code - which has been published publicly so developers can examine it - has two ranking mechanisms: Time, so that new posts are favored over old posts; and net positive or negative votes, so that posts people like are favored over those that people don't like. The problem, Greenleaf says, occurs when a new post gets a few negative votes before it gets any positive votes, rendering its vote score less than zero: ... imagine one submission made a year ago, and another submission made just now. The year-old submission received 2 upvotes, and today's submission received two downvotes. This is a small difference - perhaps today's submission got off to a bad start and will rebound shortly with several upvotes. But under this implementation, today's submission now has a negative hotness score and will rate lower than the submission from last year. Greenleaf says that the formula condemns some posts to a "purgatory" in Reddit, where they never get seen by other redditors. "These posts are sad, alone, and afraid. And notably, they are sorted oldest first, just as I predicted." Systems librarian Jonathan Rochkind has made the same claim: So it turns out there's a significant t…


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