A student asked me today, "How do they know how many people attended a large group event, for example, the Stewart/Colbert 'Rally to Restore Sanity' in Washington D.C.?" News outlets report estimates in the tens of thousands, but what methods are used to get those estimates, and how reliable are they?
One article apparently based their estimate on parking permits... but what other techniques do we have? Please note I am not talking about capture/recapture experiments or anything of the like.
I don't have any idea. I would guess in advance that there aren't specific methods for something like this, and whatever's there is very ad hoc (such as how many parking permits were sold). Is this true? For purposes of national security - of course - it would be possible to have an analyst sit down with satellite photographs and physically bean-count the number of people there. I doubt this method is used very often.