One, two, three, four... Monday 28 March 2011
One, two, three...
One, two, three, four... The number of protesters in London on Saturday has been estimated anywhere from 250,000 to 500,000. So is it possible to count?
No one knows exactly how many people joined the march and rally against spending cuts.
As the march finished in Hyde Park, the Trades Union Congress, which organised the event, estimated there was about 250,000.
On the following day, they were saying it was between 400,000 and 500,000. So how do they count?
"It's an inexact science, which is why we're not making any very, very accurate claims," says Nigel Stanley, head of TUC campaigns.
"We have been piecing together information from yesterday and think we're close to half a million people."
The answerMeasuring capacity of streets where crowds assemble and applying formula according to how densely packed it isAllowing for the dynamic nature of a march by working out how long it takes to pass different pointsThe police get an idea simply from experience
There are two ways of estimating numbers, he says. One is drawn from having a rough idea in advance of the capacity of streets in the area where people gathered for the start of the march.
"We know that the whole of Victoria Embankment was full but backing up to Lower Thames Street and people were in all adjacent roads and across the bridges," says Stanley
This advance planning involves studying very precise maps with measurements of streets, and then multiplying the width by the length to get the area, he says. Then you apply a rule of thumb that three people per square metre is comfortable and four is like a rock concert. On Saturday, it was probably about 3.5 per square metre, organisers estimate.
This calculation, helped by having a TUC representative in the police control room looking at CCTV, prompted the early and more modest estimates, says Stanley.
But then a second stage of calculations is required, to allow for the fact this is a moving group, with many people joining the march late. This requires some knowledge of how long the march takes to pass a particular point in the road.
"We know it took more than four hours to pass any point, and that the crowd was quite dense. A combination of these two ways, plus conversations with the police and others experienced in this, led us to revise the figure from 250,000 to nearer 500,000."
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It would be better to always supply a caveat of a 20% margin of error but the media demand for accurate numbers is an impossible one to meet, he adds.
Of course, organisers of any demonstration are always open to accusations they inflate the numbers for their own ends.
That's why the police sometimes estimate numbers too. At a demonstration in London in 2003 against the Iraq War, organisers said nea
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