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In Rajar's tests, the Gfk/Telecontrol MediaWatch identified only 32% of listening, whereas the Arbitron Portable People Meter managed 59% and the Eurisko Media Monitor achieved 70%.

Rajar said the results reflected the fact the Eurisko model was calibrated to recognise less than one minute of listening, whereas the Arbitron model was not. The two performed almost identically at around 59% if these measurements were excluded.

The three models were tested in November for reliability in identifying 33 different radio services and three broadcast platforms - FM, AM and DAB - under a variety of real-life situations. These included different background noises, different volume levels and wearers being stationary and on the move.

Sally de la Bedoyere, the Rajar managing director, said the performance of the Arbitron PPM and the Eurisko Media Monitor was "in line with that anticipated when designing the test".

She said GfK was making improvements to its technology and Rajar would be happy to review the situation, if appropriate, in the future.

GfK/Telecontrol's model has been used since September 2003 to provide an alternative audience measurement survey to Rajar's system, funded by The Wireless Group. 

MacKenzie, who is chairman and chief executive of TWG, continues to claim that Rajar's system fails to accurately measure the audience of his TalkSPORT station. However, he failed last year in an attempt to sue Rajar for failing to introduce an audiometer system.

A spokesman for TWG said: "We don't care which technology is used as long as it is not based on diaries and people's memories."

He said TWG would continue to fund the GfK survey but would not confirm whether TWG would renew its contract with GfK, which runs until early 2006.

Rajar has committed to using the diary system until the end of 2005 and is working towards the introduction of an audiometer-based methodology by 2007. It is now drawing up plans for the next round of audiometer testing.

Arbitron's PPM system works by inserting an inaudible code into a broadcast transmission at the radio station, which the audiometer captures and then downloads to a central computer.

The GfK/Telecontrol MediaWatch and the Eurisko Media Monitor both sample the sounds to which listeners are exposed and compares these to broadcasts recorded by a central monitoring station.

The test in November was designed and monitored by the National Centre for Social Research.

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