How to balance number of ratings versus the ratings themselves?

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+6 Q:

How to balance number of ratings versus the ratings themselves?

Hello guys,

For a school project, we'll have to implement a ranking system. However, we figured that a dumb rank average would suck: something that one user ranked 5 stars would have a better average that something 188 users ranked 4 stars, and that's just stupid.

So I'm wondering if any of you have an example algorithm of "smart" ranking. It only needs to take in account the rankings given and the number of rankings.

Thanks!

+4 A:

Here's a decent article describing the theory behind a Bayesian Ranking system:

Bayesian Rating - how to implement a weighted ranking system

Justin Niessner 2010-03-22 20:35:46

Thanks, I'll be reading that.

zneak 2010-03-22 20:36:55

This answer is good. It could be made nigh perfect by copying some of the linked information over to this answer, so that it would remain if the original link dies.

Beska 2010-03-22 21:02:50

@Beska I'm a bit short on time at the moment, I'll definitely move some across shortly.

Justin Niessner 2010-03-22 21:06:30

Have a look at Code Project Rating and Reputation FAQ, it may help.

Sameh Serag 2010-03-22 20:45:52

Either I got it right and they're doing a simple weighted average (in which case this unfortunately won't help me), either I didn't understand and you'll have to explain me.

zneak 2010-03-22 22:20:03

A simple solution might be a weighted average:

sum(votes) / number_of_votes

That way, 3 people voting 1 star, and one person voting 5 would give a weighted average of (1+1+1+5)/4 = 2 stars.

Simple, effective, and probably sufficient for your purposes.

KarstenF 2010-03-22 20:55:58

How is that different than a regular average? Also, It still has the problem the OP wanted to fix, in that a person with a single vote of "5" will rank higher than a another person with 100 votes of 4.

EJB 2010-03-22 21:00:05

This is exactly what (s)he didn't want. 1 person 5 stars would give an average of 5 stars.

Beska 2010-03-22 21:00:06

@E.J. Brennan: "Jinx!"

Beska 2010-03-22 21:00:50

Wow my brain must be on holiday! Don't know what I was thinking... downvotes are deserved!

KarstenF 2010-03-22 21:24:26

Well, yeah, that doesn't really solve my problem. :/

zneak 2010-03-22 22:14:21

+5 A:

Take a look here: How not to sort by average rating.

Svante 2010-03-22 21:09:31

+1. See also Randall Munroe's article on how it's used on Reddit: http://blog.reddit.com/2009/10/reddits-new-comment-sorting-system.html . This system also has the advantage that you don't need to know the average rating, or recalculate when it changes, which is required by the bayesian system.

Nick Johnson 2010-03-22 21:56:00

This is pretty cool, though it seems to work only for a good/bad rating, and not for a "start-based" system. Might come in handy anyways, so I'll +1 you.

zneak 2010-03-22 22:12:45

@my other comment: where was my head when I wrote this? It's a "scale-based" system, not really a "star-based" one, and even less "start-based".

zneak 2010-03-23 00:19:10

Really neat, but do you have any idea how to adapt it to a star-based system ? Even though I am not sure that a star-based system is actually better that a simple +/-, I often have a hard-time picking between 3.5 and 4....

Matthieu M. 2010-03-23 16:27:54

ansaurus

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How to balance number of ratings versus the ratings themselves?

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