If big crowds meant big wins at the polls, former Vermont Gov. Howard Dean and former Texas Rep. Ron Paul would both be president. But election after election, crowd size has been an unreliable predictor of winning.

“The people at the rally are not a random or representative sample of the electorate,” says Lynn Vavreck, a professor of political science and communication studies at University of California, Los Angeles. “These are strategic and well-planned events. This isn’t just happening.”

Rallies are designed to look like spontaneous displays of excitement for a candidate, but they are planned right down to picking a venue that will overflow rather than look half-empty.

Early on, when Sanders was only polling single digits and big crowds started showing up, they were seen as proof of the appeal of his insurgent message. Now that Sanders is well-known, the rallies give his supporters a feeling that they are part of a movement, something larger than themselves.

A Sanders adviser says the events, which Sanders insists on calling “meetings,” play to the candidate’s strengths and energize the campaign in cities where the rallies take place.