Fifteen years ago, Julie Rehmeyer was a science journalist leading an active, outdoorsy life in New Mexico. She ran marathons, biked regularly, and taught mathematics and classics at St. John’s College. Just outside Santa Fe, on a parcel of streamside land, she had even built her own house—a straw-bale construction shaded by ponderosas, meant for the family she hoped to have one day. Then, over a period of a few years, Rehmeyer lost most of her strength, endurance, and confidence, along with the ability to live a normal life. A bike ride left her bedridden. A trip to the grocery store found her using the shopping cart as a walker. At home, she could make it to her bedroom only by climbing the stairs backwards, scooting herself up a step at a time. By 2006, she was no longer able to exercise, and needed to rest whenever she wasn’t working.

“Through the Shadowlands,” Rehmeyer’s new book, chronicles her struggles since then. She attempted years of conventional and alternative medicine, moved to a different state, broke up with her partner, and, finally, cobbled together a functional life. Her condition, which affects an estimated million other Americans, goes by various names—chronic fatigue syndrome, myalgic encephalomyelitis, and systemic exertion intolerance disease (S.E.I.D.), among others. (S.E.I.D. seems to me the most descriptive term, so I’ll use it here.) The Mayo Clinic defines it as “a complicated disorder characterized by extreme fatigue that can’t be explained by any underlying medical condition.”

Rehmeyer’s writing is full of verve and curiosity, and she’s warmly attuned to how her plight is, in fact, familiar: all of us become weaker as we age, at times gradually and at times suddenly, and along the way we adapt ourselves to fit our diminished capabilities. Still, her story is a biting indictment of how we approach diseases that can’t be reduced to tidy pathologies or a uniform set of symptoms. In a way, science failed Rehmeyer. Years of clinical studies supplied little insight into her affliction, and prescribed therapies had minimal effect. Part of the issue is that S.E.I.D. is a slippery condition with no known cure, but the deeper problem lies in the methodology of clinical trials, and in the premises of evidence-based medicine.

The most prominent study of S.E.I.D. is the PACE trial, an experiment conducted on six hundred and forty-one patients in the United Kingdom. The study was published in 2011, in the English medical journal The Lancet, and reported improvements in fatigue and physical function under two treatments: cognitive behavioral therapy and graded exercise therapy. These findings were used to reinforce recommendations by Britain’s National Health Service. But, in the years since, the PACE trial has been sharply criticized by both scientists and patients. One of their chief objections is that the PACE research team, while collecting its data, changed the main metric of “recovery” from objective measures, such as fitness and employment status, to subjective ones, such as the patients’ self-evaluations over time. As Rehmeyer points out, this meant that the study could claim recovery even when a patient’s walking ability was considerably worse than that of a healthy elderly adult. (The PACE pool had an average age of forty.)

The study’s conclusions were also easy to misinterpret. In a given trial, there’s always variation: some people get better, some get worse, and others improve on some measures and decline on others. With S.E.I.D., where the treatments are speculative and the condition itself is not clearly defined, it makes sense that cognitive behavioral therapy and graded exercise therapy would help at least some subset of people. But we should be careful when using averages to make inferences about causes and effects on individuals. S.E.I.D. is a diverse condition—indeed, it could very well be a set of conditions, amenable to various treatments but lumped under a common diagnosis. The success of some strategies for some percentage of people does not at all contradict the idea that many others need a lot more. As Simon Wessely, a contributor to the PACE study and a pillar of the English medical establishment, noted, “there were a significant number of patients who did not improve with these treatments . . . PACE or no PACE, we need more research.”

The controversy over the PACE study reflects a larger crisis in public health. Traditionally, the controlled trial has been considered the gold standard of medical evidence: you gather a bunch of patients and randomly assign some to a control group, which receives no treatment, and some to an experimental group, which does. You then compare the outcomes, and if the difference is large enough, exceeding the bounds of chance or natural variation, the result is said to be statistically significant. In designing the experiment, you want enough people in your study that you are likely to find a statistically significant difference; this is called “high power.”

There are two key problems with this approach. First, what happens when a treatment helps some patients and not others? Average improvements are, and should be, relevant to organizations such as the N.H.S., but only individual or subgroup analyses can reveal the full range of effective therapies. This, however, leads us to the second problem: if you limit your investigation to a smaller group, then it will have less statistical power. The conclusions drawn from studying subgroups are inherently noisier and likelier to mislead—a fact that doesn’t pair well with researchers’ desire for snappy conclusions and clear-cut results. Though the controlled trial nicely demonstrates the efficacy of strong treatments on well-defined conditions, it falls apart, from a statistical perspective, when applied to a phenomenon that, in Rehmeyer’s words, “science doesn’t understand.”

For conditions like S.E.I.D., then, the better approach may be to gather data from people suffering “in the wild,” combining the careful methodology of a study like PACE with the lived experience of thousands of people. Though most may be less eloquent than Rehmeyer, each may have his or her own potential path to recovery.