Why

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 ...