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2018 House · IL-12

The New York Times/Siena College · fieldwork ended 2018-10-20 (17 days before the election) · 502 respondents · Live Phone

The poll said

Democrat 39.36% · Republican 47.87%

R+8.5

The certified result

Democrat 45.39% · Republican 51.57%

R+6.2

The arithmetic

  1. Poll margin = 39.36 − 47.87 = -8.51
  2. Actual margin = 45.394 − 51.572 = -6.178
  3. Signed miss = -8.51 − (-6.178) = -2.331
  4. Absolute miss = |-2.331| = 2.331

The poll overstated the Republican by 2.3 points and had the right leader.

Where these numbers come from

Poll and result: 538 raw_polls.csv. Poll ID 56322, race ID 314 (538 database).