readability and usability which I have covered in previous posts, a third aspect of open data is accessibility.
The purpose of releasing data as open data is to enable people to use your data. To use that data they have to get it from your computer to their computer. There are a variety of ways to do that, but in 2010 that means the Internet and and the HTTP protocol. If your datasets are very large, using anonymous FTP would likely also be acceptable to many developers. However HTTP is by far the simpler protocol to use. It has many advantages from a developer perspective over FTP and it is just as easy to set up from a publisher's perspective.
Accessibility is just as important as readability. Where poor readability imposes a one time cost to developers, poor accessibility actually imposes an ongoing transactional cost. As a developer, I can write scripts to decode data provided in proprietary formats like XLS or SHP (so long as that's still legal in Canada - locks today, proprietary formats tomorrow?). It's still costly in terms of my time, but once written, I can run that same script over and over again with no effort. Poor accessibilty on the other hand sometimes means that I can't readily automate the process. If I can't automate it, then every time I want to use your data, I incur cost in my time to manually download it. That may be fine for those users who only want to download your data once. But to the developers that you want to encourage to use your data as a platform to build valuable applications with, it's a barrier they won't likely cross due to the high transactional cost.
In some cases folks use login screens or mail back mechanisms to track who is accessing our government data. In some cases there are check boxes for so called "agreements" or "contracts" that are meant to force people into some sort of agreement before they use our data. The worst of course are cost recovery models where we are forced to pay for our data twice. First as taxpayers and again as users.
Open data is emerging from an era where the status quo belief was that government data had to be locked down. Whether or not that was ever true is debatable, but it's clearly not true in 2010.
When publishers realize that with open data, the data is likely going to be re-purposed and distributed in different forms anyway, and in ways that these methods won't track, then what are you really measuring that you couldn't measure with just a simple, unobtrusive web site and access log.
When we as publishers talk about accessibility, as with many aspects of open data, it's useful to remind ourselves of the reason we are doing open data in the first place. Making data accessible means making it as easy as possible for developers to gain access to and download the data. Not so you can pass some test of "openness" (although there are good reasons to do that which I will cover in a future post), but so people use your data. You want people to use your data. That's the point.