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Thursday, February 02, 2017

Making crowd estimates is a mix of science, statistics, computer analysis and guesswork | Phys.Org

Photo: Lisa M. Krieger
"It's what everyone wants to know - but is literally impossible to answer: How many people took to the streets Saturday in the women's marches across the United States?" notes Lisa M. Krieger, The Mercury News.

Crowd image contains thousands of individuals
Photo: The Center for Research in Computer Vision 

Initial reports in The Associated Press settled for the wildly inconclusive "more than 1 million." But a project this week by two professors to sum up the tallies at 680 marches across the country was pushing estimates as high as 5 million and beyond.

Never before has the size of a been so contentious - yet so darn hard to count.

Crowd counting is an imperfect, laborious and contentious endeavor, ranging from rigorous science to random guesswork. And, as President Donald Trump's weekend fit over the reported size of his inauguration turnout shows, sometimes it's influenced by politics.

Counting swarms of restless humans is like trying to estimate the size of schools of sardine in the sea or herds of wildebeests migrating across the African savanna.

Official Women's March estimates in some cities, such as Oakland, Calif.'s 100,000, relied on aerial photos and analytics. Others enlisted block-by-block head counts. San Jose, Calif., based its 25,000 count on comparisons to similar-sized crowds with reliable counts. San Francisco police, "for reasons of public safety," calculate a tally - but do not publicly disclose it.

A lot is at stake: Does a cause have widespread popular support, or not?

University of Connecticut professor Jeremy Pressman and University of Denver professor Erica Chenoweth are "crowd sourcing" figures - both high and low estimates - for marches across the world, including 554 in the U.S. As of Monday evening, their tally for U.S. marchers ranged from 3.16 million to 4.68 million. Included are reports of marches where the only source is the marchers themselves - such as 11 marchers in Beaufort, N.C.

"Accurately counting things is hard to do. Producing defensible estimates is hard to do," said Steve Doig, a data journalist expert at Arizona State University, who studies the techniques and challenges of crowd counting. "The only thing you can do is show your work, to show how you did it."

In controlled environments, like a sports stadium, it's easy. "You have turnstiles and tickets sold," said Doig. "But for things that are impromptu or outdoors, with many entry points and many people milling around, further back ... that's much harder."

There are plenty of indirect measures of a crowd: How many people rode subways? How many buses were lined up? How much trash accumulated? How many portable toilets were used?

But direct measurements are far tougher, in sprawling crowds of varying densities. At the Women's March in Washington, D.C., there were brigades of banner-holders, as well as people in wheelchairs and strollers. Crowds spilled out into side streets and alleys. Many children saw the march from the shoulders of their parents. And there were pole and tree-climbers, perched high above the National Mall.

At the same time, Trump lashed out at the media for what he insisted were low estimates at his inauguration.
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Additional resources
Crowd counting - Wikipedia, the free encyclopedia
Counting in Extremely Dense Crowd Images - The Center for Research in Computer Vision

Source: Phys.Org