Sandbox

Statistical frustration

The headline of Gina Kolata’s New York Times article is “Spinal-Fluid Test Is Found to Predict Alzheimer’s.” The abbreviated reprint in the Minneapolis Star Tribune is “Spinal test can detect Alzheimer’s accurately.” Someone who is not connected with the study in the Archives of Neurology says, “This is what everyone is looking for, the bull’s-eye of perfect predictive accuracy.”

Naturally, a closer look is less positive. The only numbers in the abbreviated reprint were: “The new study included more than 300 patients in their 70s, 114 with normal memories, 200 with memory problems and 102 with Alzheimer’s disease.” That is, a non-NYT reader would know only how many were in each arm of the study and no idea of the numerical results.

The NYT reader would find an additional paragraph (actually six in all): “Nearly every person with Alzheimer’s had the characteristic spinal fluid protein levels. Nearly three quarters of people with mild cognitive impairment, a memory impediment that can precede Alzheimer’s, had Alzheimer’s-like spinal fluid proteins. And every one of those patients with the proteins developed Alzheimer’s within five years. And about a third of people with normal memories had spinal fluid indicating Alzheimer’s. Researchers suspect that those people will develop memory problems.”

Discussion Questions

1. The word “accuracy” is ambiguous because of the different types of errors in medical testing, Prob(no disease|test+) and Prob(disease|test-). Noting that no efficacious treatment exists at present, which error seems more serious?

2. From the results stated in the NYT, does “bull’s-eye of perfect predictive accuracy” seem warranted?

3. The Minneapolis Star Tribune cut out the final six paragraphs of Kolata’s NYT article. Give a justification and a criticism for the abbreviation.

Submitted by Paul Alper