OK, this post is only tangentially related to astronomy so feel free to stop reading now if you want to. For some time I have been worrying about ranking algorithms, mainly in the sphere of podcasting but in a wider context too. This is partly because I use some in the Astronomy Media Player and I'm worried that they aren't fair. However, as the AMP has relatively few people using it I'm aware that I may be suffering from problems with low-number statistics. So, I'll illustrate the problem with something that has more users.

For the last few months I've been monitoring the "Top Podcasts" under the Science & Nature and Natural Sciences sections in the iTunes UK store and wondering how it gets calculated. I haven't been doing this systematically (because it seems impossible to get data from iTunes without typing by hand), but I have noticed a distinct trend. Whatever gets "featured", does well in the rankings. If you've never used iTunes, you probably don't know what I'm talking about (it doesn't seem to be web-accessible), so here is a screenshot to illustrate the point.

iTunes rankings
Screenshot of iTunes UK Natural Sciences section on 19/5/2007 CREDIT: iTunes is created by Apple Computers.
The screen consists of several sections. The top section is titled "New & Notable" and this provides links to new podcasts chosen by iTunes staff (the "new" is questionable considering that the Cambridge Science Festival finished in March). The "Featured" section is presumably also chosen by Apple staff and this provides larger images and links to individual podcasts that have been around for a while. On the right are the top 25 podcasts for today. I have overlaid a red, dotted box to indicate the amount of this page displayed on a 1024x768 pixel computer screen.

It would appear that to be featured (with an image) above the fold has a very strong affect on your ranking in iTunes. Of the top 10 podcasts, only one (Brain Food) is not currently 'featured' above the fold. In fact, of the top 25 podcasts, only seven are not featured on the page at all (highlighted by red boxes). Also, one one featured podcast (Science Update) does not appear in the top 25. Of course, it could be that being in the top 25 helps to get you featured but that does not appear to be the case. Over the past week I've been watching the Jodcast's rankings and they definitely lag behind being featured (the Jodcast lept up from around 26th place two days ago to 12th after being featured). It would appear that people, understandably, generally just click on the pictures put in front of them.

So having a prominent placement on something like iTunes boosts the number of people listening to a podcast. What worries me is that because people tend to choose the 'featured' podcasts it means that the folks at Apple, who decide what gets featured, have a lot of influence over the success of podcasts. With little turnover on the featured list, it can be difficult for new podcasts to build up listeners. Obviously, Astronomy Cast managed it but many others have not. This problem isn't just limited to iTunes though, with features such as Amazon's "people who bought this also bought..." tending to become boring if there are few eclectic people in the mix.

Now back to my problem. When I first put small podcast pictures on the front-page of the Astronomy Media Player, I noticed that those near the start of the alphabet started doing much better than they had done before. I then changed the listing to be sorted by popularity, and this appears to have had a positive feedback effect; those at the top reinforce their popularity and therefore remain at the top. I don't feel too happy about this and am trying to think of a solution. Obviously, popular podcasts should generally be near the top of the list but some randomness should probably be thrown in. The randomness would add a bit of churn to keep it interesting. The only problem is finding the right level between the positive feedback and the randomness. Any comments or suggestions?

Posted in astro blog by Stuart on Saturday 19th May 2007 (15:18 BST) | Permalink
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