http://www.nytimes.com/2011/03/15/health/15letters-STUDYINGAFAT_LETTERS.html?_r=3&pagewanted=print Studying a Fatigue Illness (1 Letter) To the Editor: In Defining an Illness Is Fodder for Debate (March 8), your reporter David Tuller correctly noted that the way an illness is defined can often determine what is found in studies of it. He also suggested that this problem of definition had limited the interpretation of our trial of treatments for chronic fatigue syndrome, published recently in The Lancet. That is not the case. The patients in this trial had a disabling chronic illness in which fatigue was their main symptom and for which no alternative had been found; that is the definition of the syndrome used in Britain. But we also assessed trial participants to see if they met two other definitions of the illness that are favored by some scientists. We found that both cognitive behavior therapy and graded exercise therapy, when added to specialist medical care, were most effective not only in the whole sample but also in the participants who met these alternative criteria. In addition, these treatments were the most effective whether or not a patient was depressed, a not uncommon accompaniment to this chronic and misunderstood illness. The trial also found that these treatments were safe so long as they were provided by appropriate therapists trained to help patients with chronic fatigue syndrome. So to Mr. Tullers question Does the evidence from that study prove that these strategies would help patients identified as having chronic fatigue syndrome through very different criteria?, the answer is Yes, it does. Patients and their doctors now have robust evidence that there are two safe treatments that can improve both symptoms and quality of life, however the illness is defined. P. D. White T. Chalder M. Sharpe London and Edinburgh The writers are the principal investigators of the PACE trial. David Tuller replies: The article asked whether findings among a population defined by one set of criteria would apply to populations defined by very different criteria. In this study, all participants were first defined, identified and selected not by different criteria but by the same criteria, the so-called Oxford criteria used in Britain. Subgroups within that already screened population who also meet secondary criteria are not easily compared to patients who have not been screened, since an unknown number who met the secondary criteria might not have met the studys criteria for inclusion. The gold standard for making comparisons across groups of patients identified by three varying case definitions would be a study with three completely separate cohorts, not one large sample with two embedded subgroups. Science Times welcomes letters from readers. Those submitted for publication must include the writers name, address and telephone number. E-mail should be sent to firstname.lastname@example.org. Send letters to Science Editor, The New York Times, 620 Eighth Avenue, New York, N.Y. 10018.