What an election poll’s margin of error can and cannot explain
Sampling uncertainty is one reason estimates can move between surveys.
A margin of sampling error describes how an estimate might vary because a survey interviewed a sample. Pew Research Center notes that a poll's margin for one candidate's share differs from the uncertainty around the gap between two candidates.
The figure leaves other sources of uncertainty, including differences in how polls reach respondents and model who will vote. Reading the sampling method and comparing several polls can help readers judge whether an apparent change is larger than the variation they would expect.