Since the days when Neolithic butchers hung Stegosaurus steaks at the entrance to their caves to attract custom, outdoor advertising has been accepted as an effective medium.

But in the first decade of the twenty-first century, we are little wiser than our stone-age forebears as to which cave site attracts sales. Or why. Or when.

But that's all about to change with the advent of Nielsen Outdoor's new gizmo Npod (Nielsen Personal Outdoor Device). The cellphone-size gadget, complete with GPS (global positioning system) tucks into a pocket and accompanies its carrier wherever he/she goes.

The system keeps accurate tabs on its host's whereabouts, allowing Nielsen to match their travels to a coded map of outdoor sites in a given area.

Recently on test by around four hundred people in the Chicago area, each participating in the trial for a ten-day period, the technology lets Nielsen monitor the time and frequency of opportunities to view all sites within the locality.

The researcher will continue to gather data until the end of this month and plans to present in September a detailed demographic breakdown of Chicago outdoor-ad viewers, the data to include age, gender, income and education .

As an example of the data quality, one recent finding indicates that more than 50% of all outdoor advertising impressions among Chicago-area residents earning more than $100,000 a year happen in such neighborhoods as Calumet City, Elsdon, Melrose Park and Englewood.

But as Nielsen Outdoor's head of communications Will Thoretz points out: "These are all lower- to mid-income areas, and our findings dispel the myth that only the location of outdoor sites determines who is exposed to the advertising message."

A similar (but non-electronic) measurement system has been in use in the UK since the early 90s. Operated by London-based Postar, it has helped boost outdoor advertising from a total ad market share of 4.9% in 1999 to 9% this year.

Says John Connolly, svp for out-of-home for MediaCom USA: "I think [Npod] will double the business in less than ten years."

Data sourced from: The Wall Street Journal Online; additional content by WARC staff