Arbitron has moved up its scheduled date for increasing its cell-phone-only household samples in markets that still use diaries to measure ratings.

By spring 2010, Arbitron says 15% of its diary sample will come from households that use a cell phone instead of a landline. The increase is intended to reflect more listeners ages 18-34, who are less likely to have a landline telephone.

Arbitron had originally planned to hit the 15% cell-phone-only mark by the end of 2010. Cell-phone-only households are now measured in all markets where Arbitron measures radio listenership.

Arbitron says an average of 15% of its Portable People Meter (as opposed to diary) panel already consists of cell-phone-only households, a figure it says it will raise to 20% by the end of 2010.

Meanwhile, U.S. Rep. Edolphus Towns, chairman of the House Oversight and Government Reform Committee, threatened earlier this month to introduce legislation that would compel Arbitron to resolve its PPM-related issues if the company, the Media Rating Council (which accredits ratings) and Arbitron critics the PPM Coalition don't come up with a plan to do so in 30 days. Arbitron has been accused of underrepresenting minority listeners under the PPM system.