The proportion of those called who end up taking part in a survey has fallen steadily, from 35% or so in the 1990s to 15% or less now, according to Mr Keeter.
ECONOMIST: Political polls
2.
It helps that the sort of people who do not have landlines are also the sort who tend to turn out in relatively low numbers in mid-term elections, notes Scott Keeter of the Pew Centre.