I was reading an article about the protests here in Paris Tuesday, and it said that the organizers of the protests estimated the number of protesters around 700,000, though the police estimated 92,000. Everytime I read articles like that, the responses are like that.
Is there some science to “estimating” the number of protesters at a protest? Or is it just, “Wow, there must be 700,000 thousand people out here!” “No, it’s only about 92,000, if you ask me”?
I once did count the number of people in a protest march, at the request of the person in charge of organising it, by literally counting the peopleas they walked past a point on the route of the march. It was about 500 people, so not on the scale of 700,000, but still a fairly accurate count. However, given that people can join or leave a protest mrch like that, you really can only ever have a rough figure.
At a much larger protest that I attended, on the Mall in Washington DC, they had lots of people going round gathering names to get an idea of the size of the protest. In that case there were in the order of 1 million people at the protest – they were able to claim that quite realistically, in my view, partly because they had gathered names. But on the ground, since I walked from one end of the crowd to the other, I think the figure was that order of magnitude.
I’ve also heard of estimates being made from aerial photographs of protests, and I’d think those estimates could be quite accurate, too.
You can estimate the size of a crowd quite easily if you’ve got a good vantage point. Find an area of crowd that looks like it’s of average density, then count ten people across and ten people down. The ‘square’ you’ve just counted contains about 100 people. How many of those ‘squares’ do you need to make the whole crowd?
For bigger crowds, count 33 people across and down, and then you’ve got a block of 1000 people. And so on.
In most cases, it’s a lot like counting sheep: count the legs and divide by two.
And that, my friend, is why I will never buy a flock of sheep from you.
Damn you! That was my line and I want it back! 
Here is an article from Wired magazine that describes a photographic method of determining the size of a crowd. The writer was the person who determined the size of the crowd at the Million Man March in 1995. And here is an article from the San Francisco Chronicle that also discusses ways of estimating crowd size.
As an aside, the National Park Service used to provide crowd estimates for events held on the National Mall. But there was a fair amount of controversy when the organizers of the Million Man March claimed they had 1.5 to 2 million participants, versus an NPS estimate of 400,000. Farrakhan threatened to sue over the low number. After that, NPS decided there was little to gain by providing crowd estimates and discontinued the practice.
You just gotta love old Calypso Louie. Hit him in the face with The Wet Trout Of Reality ™ and he threatens to sue because the truth wasn’t what he wanted to hear.
I think he’s overdue for a trip back to the mothership…
They used to provide estimates for the annual Fair St. Louis held on the grounds of the Gateway Arch. In that case, there was a lot of criticism that the NPS was overestimating the crowd. I guess they finally decided it’s a no-win situation either way.
William Poundstone wrote an article about crowd estimates in one of his Big Secrets books and showed the discrepancies between sizes estimated by the organizers and the theoretically impartial folks (NPS, police department, etc).
You can do some quick math to figure out if a number is reasonable. Suppose you pack people pretty tightly together and can get one person per two square feet.
One million people would be a group packed that way 100 feet wide by a mile long. Take a look at the space that they were supposed to have occupied and see if that’s in the realm of possibility.
The National Mall is 146 acres, about 6.6 million square feet. If you packed it side to side and front to back with people (keep in mind we’re talking “no room to move” packing) then you could fit a little over 3 million folks in there.
So if you see a photo of the national mall and the first third of it is occupied by people with plenty of room to move, someone claiming 2 million attendees is probably way off.
Clothahump. I’m not sure what your response has to do with this thread. If you want to add some factual information to help answer the OP, then do it.
samclem
Similarly, the 1993 March on Washington for Lesbian, Gay and Bi Equal Rights and Liberation was severely undercounted by NPS. Organizers estimated 1,000,000 (and having been stuck in the back of the pack for 6 hours I can believe it) and NPS estimated 300,000. We were pretty pissed about the obvious undercount, but we didn’t AFAIK threaten to sue anyone about it.
What I’ve heard is that event organizers will almost always overestimate the attendance at their event. For large events, like the Million Man March, the error can be quite significant. Plus, the news media will usually publicize the larger estimate.
In many cases, the answer to the OP is “they can’t and they don’t”. The organisers give their estimate, and it’s (usually) in their interests to inflate the figure as much as possible. The official sources (police, news crews etc.) give their estimate, and it’s often in their interests to try and downplay the figure. But unless either side provides a detailed and verifiable account of how they arrived at a realistic figure, there’s no reason to suspect it’s anything other than a fairly arbitrary opinion with a fair amount of bias involved.
On the rare occasions that anything like an accurate figure is provided, it’s done either by ensuring all participants pass a fixed point and counting at that fixed point, or by using aerial photography and statistical sampling.