Drones - the impossibility of accurate statistics

Whilst accepting that US drone strikes in Pakistan's Federally Administered Tribal Areas (FATA) - and elsewhere - regularly result in the deaths of non-combatants as 'collateral damage', I have always argued it is impossible to be precise about the numbers.
Reports in the last year from the Bureau for Investigative Journalism, Stanford Law School/NYU School of Law, the New America Foundation and the Long War Journal all rely on figures provided to local reporters by unnamed security officials - usually members of Pakistan's ISI, who refuse to divulge their identity. Outsiders cannot check these figures as they are banned from entering FATA. The US authorities, which claim that civilian deaths are minimal, fail to provide any evidence to back up their argument.
All the various analytical reports attempt to portray themselves as authoritative, although all use different methodologies.
The debate on drone casualties is rapidly becoming a classic debate on the use and abuse of official statistics. The latest contribution comes from the Human Rights Clinic at Columbia Law School, whose report, Counting Drone Strike Deaths has produced a further set of estimates - although it makes a point of stating the impossibility of accuracy: “Drone strike casualty estimates are substituting for hard facts and information about the drone program,” says Naureen Shah, Acting Director of the Human Rights Clinic at Columbia Law School, adding: “These are good faith efforts to count civilian deaths, but it’s the US government that owes the public an  accounting of who is being killed, especially as it continues expanding secret drone operations in new places around the world.”
Columbia Law School also recently published The Civilian Impact of Drones: Unexamined Costs, Unanswered Questions, which examines the way drones are rapidly becoming the centrepiece of US counter-terrorism strategy.