From how many people attended President Donald Trump's inauguration to how many protesters filled streets during Women's Marches across the country, people are arguing over the science of crowd counting in our cities.
There's no question that on January 21, the streets of the United States—in small towns like Durango, Colorado, and in big cities like New York and Los Angeles—were full of people participating in Women's Marches. According to data collected by University of Denver international studies professor Erica Chenoweth and political scientist Jeremy Pressman at the University of Connecticut, there were more than 500 marches in U.S. cities, attended by millions of people.
But estimates of exactly how many people attended the Women's March varies depending on who you ask: A study by FiveThirtyEight found that in at least 11 cities around the country, organizers' crowd size figures topped those offered by local officials by 40 percent.
The contested nature of crowd estimates raises some questions: How do people count crowds? Why is it so difficult? And is there ever an accurate number? With the help of Dr. John McCarthy—a leading sociologist at Penn State University—we break down these answers and reveal five things you need to know about counting crowds in cities.