I had yet another chance to do crowd-estimating today. Lisbon hosted the NATO conference here Friday and today, an event that brought out many protesters. Working again with several of my students and my colleague Antonio Granado, we came up with an estimate of the size of the main protest march that occured this afternoon.
As is the custom for such demonstrations in Lisbon, the protesters gathered at the Marques de Pombal circle, then marched down Avenida da Liberdade to Restauradores.
To estimate this crowd, we had to rely on counting the marchers as they passed by along the avenida. Our protocol was to count how many marchers passed by during 30-second periods, and taking these counts every two minutes. The reason for the protocol is to make sure that the counts were a random selection of the varying numbers of people who passed by; as happens with these marches, sometimes there are large clumps and sometimes gaps or only a few people going past. By using a rigid protocol like this, we avoid unconsciously influencing the numbers by taking counts only when dense or sparse groups were going by.
The front of the march reached my position at 3:55pm and the last of them passed at 4:40pm; thus, the march lasted 45 minutes. My 30-second counts ranged from as high as 300 at the very front of the march to as low as 10 persons during another spot. (No, I didn’t count to 300 for that first one. In fact, I counted about 150 during a 15-second period instead, a necessary fallback when a crowd is very dense. But all the other counts were made during 30-second periods.)
When the march ended, I took the average of the counts (about 68) and then multiplied by two to turn it into average marchers per minute. That answer was 135. I multiplied that by the 45 minutes of the march and got 6,750, which I rounded up to 7,000. I then added an arbitrary 1,000 to account (somewhat liberally) for marchers who may have left before arriving at Restauradores.
My estimate of 8,000 is well below the march organizers’ claim of 30,000 protesters. But there are reasons why the 30,000 claim is not credible.
One reason is this picture taken of the crowd by Antonio from above Restauradores after the last of the marchers reached the plaza.
The area filled by the crowd is about 5,000 square meters. A crowd with this density takes up about 0.7 square meters per person; in this case, that means the crowd area would hold about 7,000 people — not 30,000.
Those who believe the count of 30,000 is accurate cite the fact that the marchers extended the entire length of Avenida da Liberdade; as the front of the march reached Restauradores the last of the march was just leaving Marques do Pombal.
But Avenida da Liberdade is 1,100 meters long, and the street area used by the marchers is 20 meters wide. Therefore the rectangle which contained the march is 22,000 square meters. If every one of the marchers were tight enough to reach out and touch the person in front or next to them, they would use about one square meter per person. Thus it’s possible to have just over 20,000 marchers in Avenida da Liberdade at one time. But this march was not that packed together; as noted above, there were tight clumps and there were empty stretches.
So 30,000 is not credible. For the reasons cited above, I believe that an estimate of about 8,000 is realistic.
(POSTSCRIPT: Some will note that today’s crowd clearly was larger than the trade union march and rally of Nov. 6, and yet I estimated 8,000-10,000 at that event. I have to admit now that I was being overly generous in that earlier estimate. The crowd picture taken then showed maybe 4,000 people, and I doubled that estimate in order to account — way too much, I now believe– for those who wandered away before the march ended. All of which goes to show the importance of being not only realistic in these estimates, but consistent!)
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