The company had previously committed to a target of 15% by the end of 2009, but has been accused of not keeping up with the increasing number of households ditching their landlines, already estimated as about 18% of US homes.

It has also brought in a 15% target for areas where it still conducts diary-based measurement, to be met by the end of 2010. Cell-only sampling will be included in all diary markets by this autumn, the firm said.

Under pressure from clients, the firm’s targets have crept gradually upward ever since it first began sampling cell-onlys last autumn. Taking steps to include homes without landlines in research is considered increasingly important as more and more people ‘cut the cord’, particularly because many of those cord-cutters are from hard-to-reach young and ethnic-minority demographics.

But reaching cellphones for research is difficult because of high costs, auto-dialling rules, people’s reluctance to take non-personal calls and the difficulty of identifying participants who have a cellphone but no landline.

Jay Leve of SurveyUSA recently cited the rise of cell-onlys as a factor contributing to the decline of telephone research.

The latest changes to Arbitron’s targets follow a meeting with the firm’s Radio Advisory Council, which includes representatives from its customers in the radio industry.

• Consumer and media research firm Scarborough – Arbitron’s joint venture with Nielsen – announced this week that it will start incorporating cellphone-only (CPO) users in its sample frame for syndicated local market studies. Beginning with 10 markets this summer, Scarborough will include CPO samples in all its top-tier local markets and its national Scarborough USA+ study by spring 2011.