A further 14.7% of homes had a landline but rarely if ever used it, meaning that about 85 million American adults are now cell-only or ‘cell-mostly’.

The NHIS report said that the potential for bias in phone surveys continues to grow. Surveys limited to landlines might still be viable for sampling all adults, or for some subpopulations, it said, but “caution is warranted when using landline surveys to draw inferences about subpopulations more likely to be wireless-only (such as young or low-income adults)”.

Radio ratings firm Arbitron has brought forward the schedule for raising the proportion of cellphone-only homes in its diary panels to 15% by six months, pledging to reach the target by spring 2010.

The 33 markets that have switched to the firm’s portable people meter (PPM) measurement system already have an average of 15% cell-onlys, it says, and it intends to increase that figure to 20% by the end of 2010.

Alton Adams, Arbitron’s chief marketing officer and executive vice president, said the firm had completed the roll-out of CPO sampling in all 50 mainland states during fall this year, and was now seeing improved sample performance in those markets.

Arbitron has previously come under fire from ethnic minority-owned broadcasters, who claimed that the firm was not keeping up with the increasing number of CPO households, causing PPM data to be “skewed and unreliable”.