# New "Best" comment sorting system

Way back in October 2009 Reddit introduced their "Best" comment sorting system. We've just pulled those changes into Less Wrong. The changes affect only comments, not stories.

It's good. It should significantly improve the visibility of good comments posted later in the life of an article. You (yes you) should adopt it. It's the default for new users.

See http://blog.reddit.com/2009/10/reddits-new-comment-sorting-system.html for the details.

## Comments (17)

Best*13 points [-]Short version of how this is different, for those too lazy to click on the link: if you sort by "top", comments get sorted in a simple "the ones with the highest score go on top" order. This has the problem that it favors comments that were posted early on, since they're the ones that people see first and they've had a lot of time to gather upvotes. A good comment that's posted late might get stuck near the bottom because few people ever scroll all the way down to upvote it.

"Best" uses some statistical magic to fix that:

Not sure I fully understood that either. But they say it works well, so I guess I'll trust them!

*6 points [-]I'm curious whether the math still works correctly on a site where the default karma is 1 instead of 0. But since it's magic to start with, I guess "meh". Let's just not use it to calculate CEV or anything. ;-)

*3 points [-]I think what they're doing is doing statistical inference for the fraction upvotes/total_votes. I'm not sure this is the best model, possible but it seems to have worked well enough.

I suspect they're taking the mean of the 95% confidence interval, but I'm not sure. There's actually a pretty natural way to do this more rigorously in a Bayesian framework, called hierarchical modeling (similar to this), but it can be complex to fit such a model.

Edit:However, a simpler Bayesian approach would just be to do inference for a proportion using a 'reasonable' prior for the proportion (which approximates the actual distribution of proportions) expressed as a Beta distribution (this makes the math easy). Come to think of it, this would actually be pretty easy to implement. You could even fit a full hierarchical model using a data set and then use the prior for the proportion you get from that in your algorithm. The advantage to this is that you can do the full hierarchical model offline in R and avoid having to do expensive tasks repeatedly and having to code up the fitting code. The rest of the math is very simple. This idea is simple enough that I bet someone else has done it.*5 points [-]If you use the Bayes approach with a Beta(x,y) prior, all you do is for each post add x to the # of upvotes, add y to the # of downvotes, and then compute the % of votes which are upvotes. [1]

In my college AI class we used this exact method with x=y=1 to adjust for low sample size. Someone should switch out the clunky frequentist method reddit apparently uses with this Bayesian method!

[1] This seems to be what it says in the pdf.

How do the "Best", "Popular", and "Top" algorithms work?

Ironically, it appears the new algorithm is frequentist.

Bayesian reformulations welcome.

Apologies â€” I should have taken reinforcement into account and noted that the new algorithm is probably still a lot better than the previous one.

This seems like a neat problem. Would it be hard to go from a python function that takes a set of comment upvote downvote counts and returns a ranking to a comment sorting option? If I don't know much about the reddit internals?

Also, would it be difficult to get a real dataset of comment counts from LW?

*2 points [-]"Top" simply calculates the (number of upvotes - number of downvotes) and puts on top the comments that rank the highest this way.

I think "Popular" tries to favor comments that don't have many downvotes or something, I'm not sure.

"Best" apparently works by magic.

*2 points [-]I think "Popular" adds weight to

recentcomments. This seems to be a much worse way of achieving what "Best" shoots for.Yay! Thanks Matt and the tricycle team (and anyone else) for continuing to improve LW.

Work done by John Simon, and integrated by Wes.

Thanks, John and Wes!

Thanks Matt!

Thank you for taking the time to implement this, I've set it as my "sort by" criteria.

I've noticed that the "Best" sorting sometimes puts strongly downvoted comments (score -5 or less) above comments with scores closer to zero. Is this intentional or a bug?