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Why by Samantha Kleinberg

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Chapter 3. Correlation

Why are so many causal statements wrong?

In 2009, researchers found a striking relationship between a virus called XMRV and Chronic Fatigue Syndrome (CFS).1 While millions of Americans suffer from this disease, which is characterized by severe long-lasting fatigue, its cause is unknown, hampering efforts to prevent and treat it. Viruses, immune deficiencies, genetics, and stress are just a few of the many hypotheses for what triggers this illness.2 Yet in addition to the competing causal explanations, simply diagnosing the illness is difficult, as there is no single biomarker that would allow a definitive laboratory test. Many cases go undetected, and it’s possible that CFS is actually a collection of multiple diseases.3

So when that group of researchers, led by Dr. Judy Mikovits, found that of 101 patients with CFS 67% had the XMRV virus, and only 3.7% of the 218 control subjects did, people took notice. While the virus didn’t explain all cases, there could be a subgroup of patients whose CFS is a result of the virus, and some of the seeming controls could be undiagnosed cases. For a disease that had proven so difficult to understand, the numbers were remarkable and spawned a variety of efforts to try to confirm the results. Multiple studies failed to find a link between CFS and XMRV,4 but in 2010, researchers found a similar virus that also had markedly different prevalence in CFS patients (86.5%, 32 of 37) compared to healthy blood donors (6.8%, 3 of ...

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