Although landline polling remains the default for telephone surveys, as adding cellphone users increases cost and complexity, a new Pew study reveals that landline-only respondents tend to be more politically and socially conservative than respondents from mixed or cellphone-only homes.

The biases stem mostly from age, economic class and race: 18-to-29-year-olds account for 41 percent of cellphone-only ownership yet just 7 percent of landlines, while cellphone-only ownership tilts heavily toward minorities, people making less than $30,000 a year, and those who have never attended college.

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