But there are no reliable statistics at the state or local level on the share of CPOs in the population.

ECONOMIST: Political polls

CPOs are younger, less white, and poorer than the average American.

ECONOMIST: Political polls

At the national level it is easy enough, though expensive, to account for this by surveying CPOs and including them in the sample in proportion to their share of the population.

ECONOMIST: Political polls

But even if fears of a systematic bias prove unfounded, the fuss about CPOs points to a broader problem for pollsters: the ever-increasing difficulty of persuading Americans to take part in political polls.

ECONOMIST: Political polls