The Linked Data movement was kicked off in mid 2006 when Tim Berners-Lee published his now famous Linked Data Design Issues document. Many had been promoting the approach of using W3C Semantic Web standards to achieve the effect and benefits, but it was his document and the use of the term Linked Data that crystallised it, gave it focus, and a label.
In 2010 Tim updated his document to include the Linked Open Data 5 Star Scheme to “encourage people — especially government data owners — along the road to good linked data”. The key message was to Open Data. You may have the best RDF encoded and modelled data on the planet, but if it is not associated with an open license, you don’t get even a single star. That emphasis on government data owners is unsurprising as he was at the time, and still is, working with the UK and other governments as they come to terms with the transparency thing.
Once you have cleared the hurdle of being openly licensed (more of this later), your data climbs the steps of Linked Open Data stardom based on how available and therefore useful it is. So:
|★||Available on the web (whatever format) but with an open licence, to be Open Data|
|★★||Available as machine-readable structured data (e.g. excel instead of image scan of a table)|
|★★★||as (2) plus non-proprietary format (e.g. CSV instead of excel)|
|★★★★||All the above plus, Use open standards from W3C (RDF and SPARQL) to identify things, so that people can point at your stuff|
|★★★★★||All the above, plus: Link your data to other people’s data to provide context|
By usefulness I mean how low is the barrier to people using your data for their purposes. The usefulness of 1 star data does not spread much beyond looking at it on a web page. 3 Star data can at least be downloaded, and programmatically worked with to deliver analysis or for specific applications, using non-proprietary tools. Whereas 5 star data is consumable in a standard form, RDF, and contains links to other (4 or 5 star) data out on the web in the same standard consumable form. It is at the 5 star level that the real benefits of Linked Open Data kick in, and why the scheme encourages publishers to strive for the highest rating.
Tim’s scheme is not the only open data star rating scheme in town. There is another one that emerged from the LOD-LAM Summit in San Francisco last summer – fortunately it is complementary and does not compete with his. The draft 4 star classification-scheme for linked open cultural metadata approaches the usefulness issue from a licensing point of view. If you can not use someone’s data because of onerous licensing conditions it is obviously not useful to you.
|★★★★||Public Domain (CC0 / ODC PDDL / Public Domain Mark)
|★★★||Attribution License (CC-BY / ODC-BY) when the licensor considers linkbacks to meet the attribution requirement
|★★||Attribution License (CC-BY / ODC-BY) with another form of attribution
|★||Attribution Share-Alike License (CC-BY-SA/ODC-ODbL)
So when you are addressing opening up your data, you should be asking yourself how useful will it be to those that want to consume and use it. Obviously you would expect me to encourage you to publish your data as ★★★★★ – ★★★★ to make it as technically useful with as few licensing constraints as possible. Many just focus on Tim’s stars, however, if you put yourself in the place of an app or application developer, a one LOD-LAM star dataset is almost unusable whilst still complying with the licence.
So think before you open – put yourself in the consumers’ shoes – publish your data with the stars.
One final though, when you do publish your data, tell your potential viewers, consumers, and users in very simple terms what you are publishing and under what terms. As the UK Government does through data.gov.uk using the Open Government Licence, which I believe is a ★★★.