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How Do You Estimate Crowd Size?

The recent controversies surrounding the Inauguration and the Women's March on Washington may have you asking: how do you accurately estimate the size of a crowd?

By
Sabrina Stierwalt, PhD
Episode #223

Organizers of any political rally, protest, or event will always have motivation to boost their numbers, just as anyone trying to downplay the importance of an event will benefit from underestimating participation.

For example, organizers of the Million Man March in DC in 1995 disagreed with the National Park Service attendance estimate of 400,000 people so strongly that they threatened to sue. Since then, the National Park Service has been banned by Congress from using tax payer dollars to estimate crowd sizes. Other estimates put the crowd at the Million Man March at over 800,000 people. Even if the lowest estimates are true, 400,000 people was still 1.2% of the African American population in the U.S. at the time—an incredibly impressive showing.

As is often the case in science, when an exact count is difficult, it is best to take the average of multiple measurements. 

Which Crowd Estimates Should You Believe?

Counts vary as far as the recent events of the inauguration and the resistance marches the following day. The viewing angle from the inaugural dais is also known to obscure gaps in the crowd giving the appearance of a larger attendance than the real count. Thus overhead pictures of the crowd are far more accurate. However, airspace is closed during the inauguration, meaning there are only a few official aerial photographs to work with.

As is often the case in science, when an exact count is difficult, it is best to take the average of multiple measurements. Marcel Altenburg and Keith Still, two crowd scientists from the Manchester Metropolitan University in Britain, tallied the attendees at the inauguration and came up with a similar estimate to that from Steve Doig, a census expert from Arizona State University: three times more people attended the Womens March on Saturday as attended the inauguration on Friday.

The Digital Design and Imaging Service, a Virginia-based company, plans to estimate the number of attendees at the Womens March on Washington and claims they can determine a count with an accuracy of 10%. They employed a tethered aerostat, a fancy name for a weather balloon that is tied down, with a nine-lens camera allowing for a 360 degree view of the crowd. They are working to count individual heads in every photograph which may prove the most accurate way to count the numerous children on shoulders, tree climbers, and people in wheel chairs, that otherwise might get lost in rougher estimates. Their results are still pending.

Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com

Image courtesy of shutterstock.

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About the Author

Sabrina Stierwalt, PhD

Dr. Sabrina Stierwalt is an extragalactic astrophysicist at the California Institute of Technology and Adjunct Faculty at the University of Virginia.

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