A type of public-opinion poll that allows candidates to follow or “track” voters’ sentiments over the course of a campaign is called a tracking survey. For the initial survey, the pollster interviews the same number of voters on three consecutive nights — for example, 400 voters a night for a total sample of 1,200 people. On the fourth night, the pollster interviews 400 more voters, adds their responses to the poll data, and drops the responses from the first night. Continuing in this way, the sample rolls along at a constant 1,200 responses drawn from the previous three nights. Over time, the campaign can analyze the data from the entire survey and observe the effect of certain events on voters’ attitudes. See also Poll/Polling.