What did the selected poll say?
Gallup’s selected estimate put Harry Truman at 44.5% and Thomas Dewey at 49.5%. On the chart’s stated basis, that was R +5.0. The corresponding recorded outcome margin was D +4.4, for a 9.4-point difference in the margin.
What actually happened?
The three major polling organizations used quota sampling and all picked Dewey. The selected Gallup estimate favored Dewey by 5 points; the cited rounded outcome favored Truman by 4.4.
Why might the poll have missed?
AAPOR's methodological review describes how interviewer choices within quotas overrepresented more educated and affluent respondents, creating bias against Truman. Evidence status: supported.
Gallup's retrospective also attributes part of the miss to interviewing ending before late movement toward Truman. This does not establish that a late swing alone explains the error. Evidence status: institutional retrospective, not exclusive cause.
What has not been established?
Neither late movement nor quota sampling should be presented as the entire explanation. A contemporary dataset holding the same sampling process and turnout model fixed would be needed to isolate their separate contributions.
How should we use this example?
Read the actual survey and its interviewing dates, not only the famous Chicago Tribune headline. A media organization’s premature call is a separate decision from a polling organization’s estimate.
This compares one Gallup final allocated estimate with source-reported election shares rounded to 0.1 point. The plotted error is approximate raw D−R margin error. This is not the polling industry’s mean error.
What to carry forward
The sampling and timing of a poll matter more than confidence in its winner call.