A Population Health Metrics article, How Common is Chronic Fatigue Syndrome; How Long is A Piece of String?*, discusses the problem with quantifying CFS. The syndrome has no biological markers. There are no definitive tests. Defining it is often a process of eliminating what it is not, rather than what it is.
Then there is the problem that a great many doctors think it is psychological rather than biological, or physiological. How CFS is defined is subjective rather than objective.
Last year the Center for Disease Control (CDC) began a $4 million educational program and is currently involved in a study to standardize the criteria for diagnosis of CFS.
As simple as it sounds one key factor was how survey questions are asked. For example, rather than ask if a family member suffers from ‘fatigue’ the question becomes “Have you had one or more of the following symptoms for a month or more: ‘fatigue, cognitive impairment, un-refreshing sleep, muscle pain, joint pain, sore throat, tender lymph nodes, or headache’. Studies already indicate that positive answers to similar question triples.
* Note: The article is well documented with 16 sources listed in the bibliography.