One of the problems with working with large datasets — especially when you’re scraping them — is that they don’t always work the way one might think.

We’ve recently had reports that JobcentreProPlus.com turns up jobs that aren’t close to the postcode that the user entered when they started their search. We’ve done a bit of digging, and turned up two problems. Unfortunately, neither is easily fixable.

The first problem is that JobcentrePro’s website doesn’t expose very good location data. It’s often as little as “Camden Town, London” or “Sevenoaks, Kent”. For this to be useful, we need to convert it to a latitude and longitude, so we can see if it’s near the postcode you enter when you start a search.

This process is called geocoding, and it’s an inherently error-prone process. There’s often no way to tell the the difference between places with similar names. Usually, it works well enough, but sometimes, it’ll generate a result that’s unexpected: in real terms, you see a search result for a job in Glasgow when you were searching for things in London.

There’s not a lot we can do about this. If JobcentrePlus included better geographical information in their listings — like a postcode, or a latitude/longitude — we wouldn’t have to geocode things, which would be a great improvement.

Unfortunately, in this case, it gets more complicated. The second problem is that the JobcentrePlus database (which also drives their service!) doesn’t store good location data. Sometimes the location refers to the address of the Jobcentre shop. Sometimes, it’s the agency advertising the job. Sometimes, it’s the employer’s head office, but not the actual building you’d be working in if you took the job.

In summary: the way we’re forced to gather data introduces errors, and the underlying dataset has quite a few errors to begin with.

Despite this, we still think JobcentreProPlus.com is useful. Most of the time, the job will in fact be near the jobcentre, the employer’s head office or the job agency. That’s why our “distance from postcode” field defaults to 10 miles — we’re confident that that’ll be right, most of the time.

The bottom line is that the quality of our site is completely dependent on the quality of the underlying data. Until that data is better, there’s not much we can do to improve things — but we’re not too worried. From a plain reading of search results, we think we’re doing ok. This search for stuff in London returns mostly stuff that, according to the job ad, is in London.

We think it’s good enough to be useful, and that’s really our only goal.