When, exactly, did we become white trash... goes the song. Now advertising technology is set to find out.

Or at least, when a specific shopping decision was taken. A new metric called the Location Conversion Index (LCI) is the product of ZenithOptimedia and mobile audience intelligence partner NinthDecimal.

In an industry first, the technology sets out to deliver measurement metrics on whether mobile and online campaigns were responsible for driving foot traffic to stores.

As a result, the advertising industry hopes to find out more about what's working: Which parts of a digital media campaign including mobile and web ads and social media, pushed a prospective buyer over the line.

"Since engagement rates don't equal conversion rates in drive-to-location campaigns, marketers need accurate and scalable data in order to truly understand the path to purchase," says a NinthDecimal spokesman.

LCI is being rolled out to more than 75 partners including agencies and independent ad networks, rolling out to AOL and Yahoo.