Counting crowds has a long tradition of controversy and inherent inaccuracy. Published estimates of crowds at President Obama's inauguration, for example, ranged from 800,000 to four million.
Clearly, when it comes to such large numbers of people, no one can count every head. So is it even possible to make reliable empirical estimates? It is if you do it the way doctors determine whether men are infertile.
An average male in the United States produces about 60 million sperm per milliliter of semen. No one, of course, counts all the sperm. Rather, doctors count the number of sperm in a minute sample, which is assumed to be representative. The density of sperm can then be multiplied to estimate the amount in any given volume.
Applying a similar method using detailed aerial and ground photographs of the entire site at noon (when it was thought attendance would peak), Steve Doig, an Arizona State University journalism professor who specializes in quantitative research methods, made density estimates in eight different areas of the crowd. He then used Google Earth to estimate the square footage of the various regions. He came up with crowd sizes in the different areas, which he added up over the whole rally for an attendance estimate of 80,000.
Ryan Shuler, an image analyst for AirPhotosLive, the research company that supplied the crowd photographs, used the same pictures and a slightly different method to make his estimate. He created a grid of small squares on the photos, counted individuals in a selected sample of squares, and came up with a count of 87,000.
