With the inauguration of the 45th president of the United States this past weekend and the dissenting marches that followed, there’s one type of science that is dominating the news cycle, and that’s crowd science, or the science behind estimating the number of people in a large crowd. Crowd scientists can have a range of expertise, including census work, remote sensing, geospatial analysis, and even cartography, all of which can help with the daunting task of crowd counting.

Some reports quoted the inaugural attendance as low as 250,000 people, but the president himself said “it looked like a million, a million and a half people.” Twenty-four hours later, estimates for attendees of the Women’s March on Washington were consistently larger, although they ranged from 500,000 to 1 million people. So who is right? How do crowd scientists estimate the number of people at these large events and why don’t they agree?


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