Collaborative filtering: comparing Reddit's karma system to Digg

In recent weeks I've given Digg.com a bit of coverage, albeit of the kind they wouldn't have enjoyed. But actually I love the concept of digg.com and I still think it has a lot of potential, as long as they reduce the groupthink and address the spam issues (which to be fair, are probably scaling issues more than anything). Another community news site that crossed my radar recently was reddit.com - and it may have a jump on Digg when it comes to avoiding groupthink and spam. It does this with a user reputation system it calls 'karma'.

Reddit co-founder Alexis Ohanian told me via email that Reddit is "a social news site that's not limited to tech news" and that it has "a collaborative filtering system that aims to give users not just new and popular links, but also personally relevant ones." That last bit is interesting - Reddit wants to give users personally relevant links. The way Reddit is currently going about this is via something called a karma system. Alexis says this is unique to Reddit, although similar to Slashdot's reputation system. It's cumulative and apparently "highly sought-after". This is how karma is described in the Reddit FAQ:

"When a particular item is promoted or demoted, the user who posted it is either rewarded or punished -- a system of editorial karma. In the same way that popular submissions are voted to the top, the individuals who post them get increases in karma. Every redditor affects one another's karma equally, regardless of his/her karma. Although democracy isn't perfect, this experiment should supply the public with the information they demand while also rewarding those who provide it."

Essentially it's a peer ranking system and Reddit is hoping it'll differentiate them from Digg - and perhaps get Reddit some of Digg's huge user base. While Digg doesn't have an explicit reputation system, it does give users rankings according to how many homepage diggs they get. The top diggers page lists the users who have the most homepage diggs and also lists their 'homepage ratio'. Digg's system does seem to be a populist method of ranking users - because users only get a good ranking if their stories prove to be popular enough to make the Digg homepage. So perhaps Reddit's karma system is something that Digg should look at to reduce groupthink and spam?

Indeed Reddit's Alexis Ohanian says that "with reddit, we're hoping that by focusing on filtering, users will be inclined to vote up links that genuinely interest them". The Reddit method then is trying to capture that elusive social software principle of getting the user to reward him or herself first and foremost, but actually the system is enhanced at the same time. As Alexis said, "The nice thing about this is that although users are serving themselves by voting to train a personal filter, the by-product of their honesty is that the community gets a more accurate idea of what's really popular."

What do you think. Is Reddit's karma system a better - more honest - way to rank stories and users than Digg's? Or do you think Digg has the right approach, but just needs to address the groupthink and spam issues that come with scaling to thousands of users?