The Will Rogers phenomenon: the effect of different diagnostic criteria.

The 'Will Rogers phenomenon' is an apparent epidemiological paradox named after a remark made by the humorist Will Rogers about migration during the American economic depression of the 1930's: "When the Okies left Oklahoma and moved to California, they raised the average intelligence level in both states." In 1985, Alvan Feinstein proposed the name 'Will Rogers Phenomenon' to describe the 'stage migration' he observed in patients with cancer. Changes in the criteria for assigning patients to the various stages of a disease can produce spurious improvements in stage-specific prognosis, even though the outcome of individual patients has not changed. In oncology, new imaging tools allowed detection of cancer metastases before they became evident clinically. In consequence, more patients are classified into the more severe metastatic disease stage from the less severe single tumour stage. Such a 'stage migration' resulted in an improved survival of patients in both the less and the more severe disease stages. Multiple sclerosis is also subject to the Will Rogers phenomenon since the introduction of the imaging-assisted McDonald criteria. Given the sensitivity of magnetic resonance imaging for detecting disease activity, which is believed to be five to ten times greater than that of clinical assessment, the McDonald criteria are expected to allow earlier diagnosis, hence the 'stage migration' relative to the previous Poser diagnostic criteria. Because of the ethical problems associated with carrying out placebo-controlled trials in multiple sclerosis owing to the wider availability of therapeutic options, the use of historical controls groups remains an attractive option. However, the Will Rogers phenomenon, which is recognised as one of the most important biases limiting the use of historical controls groups in experimental treatment trials, compromises the interest of this approach. In this context, the use of different diagnostic criteria may generate spurious improvements in the medium-term prognosis of multiple sclerosis, which may be wrongly interpreted as treatment effects.