Measuring many things rests on the ability to observe or collect the data. But, a number of organizations have found that they can’t keep up with the actions of ISIS:

He and his colleagues have (alone among wire services) built up a detailed spreadsheet total of civilian and combatant casualties, but faced with the near impossibility of verifying multiple daily reports of massacres in provinces rendered inaccessible since the early weeks of ISIS’s June offensive, they now largely restrict its use for internal purposes.Officials in UN’s Iraq mission (UNAMI) are similarly downbeat about the accuracy of their records.

“Since the armed conflict escalated, I would say that our figures are significantly under reported,” said Francesco Motta, Director of UNAMI’s human rights office.

“We are getting hundreds of reports in addition to those we verify that we are just simply not able to verify owing to our limited access to areas where incidents are taking place,” he added…

It’s the sheer magnitude of the slaughter that’s overstretching these groups’ resources, but ISIS’s murderous approach to the media has compounded the problem. On top of the much publicized recent beheadings of two American journalists, ISIS also has killed dozens of Syrian and Iraqi reporters. Body counts rely heavily on local news articles for coverage of incidents in towns and rural pockets far from Baghdad, and the jihadists’ seizure of up to a third of Iraq has complicated attempts to report within their areas of control.

It may be a macabre task but an important one. As the article goes on to note, this matters for political ends (different sides will spin the available or estimated numbers in different ways) and for public perceptions. In fact, social problems are often defined by the number of people they affect. Higher numbers of deaths would tend to prompt more reaction from the public but overestimates that are later shown to be false could decrease attention.