Candidates for Congress have aired more than two million television ads this year. They will have spent more than $3.6 billion. And yet there is still a small set of registered voters who are unsure about their votes in races that could pivot control of the Senate.

Speculation abounds as to whether there is anything to glean about these yet-to-decide voters. Since July, The Upshot has been working with CBS News to run the largest nationwide survey of midterm elections ever fielded. Data from the last wave of The New York Times/CBS News Battleground Tracker, collected by the survey research company YouGov, shines a bright light on undecided voters, typically too small a portion of any opinion poll to study reliably.

The battleground tracker pulled in data on more than 200,000 registered voters from around the country. Done in four waves, it provides powerful illustrations of how voters move into and out of undecided categories over the course of an election year. These movements reveal an interesting and familiar pattern about the choices that undecided voters eventually make: They’re predictable.

The data give us insight into Senate races in eight competitive states: Colorado, Georgia, Iowa, Kansas, North Carolina, Arkansas, Alaska and New Hampshire. Roughly 10 percent of the registered voters in the battleground tracker report were unsure about how they would mark their ballots. This number is relatively constant across the months of summer and into the fall campaign, and didn’t change much depending on whether we asked people in hotly contested races or noncompetitive ones (there are slightly more undecided voters in noncompetitive races).