Picture by Victoria (Mouse World)

I’ve been working on a guide to data APIs, and making decisions about what to include has forced me to think about exactly what I look for. If you’re going to build an API that’s useful to a wide range of people, and will add value to the whole data ecosystem, here’s what you need.

• Free, or self-service signup. Traditional commercial data agreements are designed for enterprise companies, so they’re very costly and time-consuming to experiment with. APIs that are either free or have a simple sign-up process make it a lot easier to get started.

• Broad coverage. There’s been quite a few startups that build infrastructure, and hope that users will then populate it with data. Most of the time, this doesn’t happen, so you end up with APIs that look promising on the surface but actually contain very little useful data.

• Online API or downloadable bulk data. Most of us now develop in the web world, so anything else requires a complex installation process that makes it much harder to try out.

• Linked to outside entities. There has to be some way to look up information that ties the service’s data to the outside world. For example, the Twitter and Facebook APIs don’t qualify because you can only find users by internal identifiers, whereas LinkedIn does because you can look up accounts by their real-world names and locations.

The first three principles are just about ease of use, but having linkable data is essential if you’re going to allow developers to innovate by combining data sources. Once you’ve got an external reference point, we can join information to come up with insights you’d never expect.