Supporters of Kamala Harris descended on DC on Tuesday evening to attend the Democratic presidential candidate's speech at the Ellipse. They lined up at sunrise to secure a spot in the crowd that gathered tightly around the stage as night fell.

The Harris campaign reported a turnout of 75,000 people, nearly double the 40,000 projection stated in the National Park Service's permit for the event. Crowd size has become a topic of intense political interest in recent years. What do we know about how to accurately calculate them as the presidential race enters its final phase of rallies before Election Day?

Jay Ulfelder is a political scientist who conducts research for the Crowd Counting Consortium, a database run by Harvard University's Ash Center for Democratic Governance and Innovation to track voter turnout at protests and other political events across the country. The database grew out of an effort to estimate the number of participants at the Women's March on Washington in January 2017 – and continues to serve as a tool to help scholars assess America's political interests. “The number of people participating provides a very rough measure of the intensity of concern about a particular topic,” says Ulfelder.

When it comes to estimating crowd sizes, including at campaign rallies, the CCC does not place emphasis on an exact number of people. “It's more of a larger scale thing,” Ulfelder said, meaning an educated guess is in order. His team considers venue capacity, information from firefighters and law enforcement, and reports from on-site journalists to come up with approximations. If the self-reports from each candidate's campaign are comparable to these numbers, the researchers include them in their data analysis. They give both the high and low ends of these estimates and then average both numbers to get “a conservative estimate of crowd size.”

Based on that methodology, here is Ulfelder's analysis of Harris' DC rally: A federal official told NBC that 40,000 people went through security to enter the Ellipse and another 20,000 were in the overflow area at the Washington Monument. The CCC is therefore based on a “low estimate” of 60,000 participants. Averaged with the highest estimate available – the Harris campaign's figure of 75,000 – the organization will put its “best estimate” at 67,500. That means the Harris campaign’s numbers “are within the right range,” says Ulfelder.

In collecting this information, the CCC will consider volume estimates provided by most political campaigns – with one major exception. “We saw this consistent pattern when the Trump campaign put out numbers where every journalist, eyewitness, everyone said, 'This is ridiculous,'” Ulfelder says. For example, Trump claimed this during an appearance with Andrew Schulz Flagrant Earlier this month, he reported on the podcast that 100,000 people attended his Oct. 5 rally at the site of his July assassination attempt in Butler, Pennsylvania. The Secret Service told CBS that the number of participants was around 24,000.

Due to concerns about the “integrity of the data,” the CCC decided to no longer include the Trump campaign’s turnout reports in its voter turnout estimates. “If Trump gives the number, we will ignore it because he has simply not made himself credible as a mass estimator,” says Ulfelder. “But in these cases, we will have other numbers to fall back on,” such as venue capacity and law enforcement numbers.

What's the point of keeping track of crowd size anyway? According to Ulfelder, the CCC's intention is not to establish a connection between voter turnout and the chances of winning the election. They just keep track of what is “descriptively interesting.” Some key takeaways from this campaign season: Since Harris took over from President Joe Biden as the party's nominee, Democratic political rallies have increased, while Trump's own rallies are on average less attended than during the 2020 campaign season.

But Ulfelder says it's important to remember that average viewership doesn't necessarily say much about a candidate's popularity — a whole host of factors could play a role in that discrepancy. For example, less attended rallies could simply be booked at smaller venues, he says. Finally, secondary school statistics prove useful: correlation is not the same as causation.

“The average size of a Kamala Harris rally since she was named the nominee has been quite a bit larger than the average Trump rally size this year and also the Trump rally size since Harris was named the nominee,” Ulfelder says. “What does that tell you? I don't know.”