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Do Physicians Understand Cancer Screening Statistics? A National Survey of Primary Care Physicians in the United States

Odette Wegwarth, PhD; Lisa M. Schwartz, MD, MS; Steven Woloshin, MD, MS; Wolfgang Gaissmaier, PhD; and Gerd Gigerenzer, PhD
[+] Article, Author, and Disclosure Information

From Harding Center for Risk Literacy, Max Planck Institute for Human Development, Berlin, Germany; the Dartmouth Institute for Health Policy and Clinical Practice, Dartmouth Medical School, Dartmouth, New Hampshire; and the Veterans Affairs Outcomes Group, White River Junction, Vermont.

Grant Support: The Harding Center for Risk Literacy, Max Planck Institute for Human Development, funded the study and provides support for Drs. Wegwarth, Gaissmaier, and Gigerenzer. Drs. Schwartz and Woloshin were supported by the National Cancer Institute (grant R01 CA104721).

Potential Conflicts of Interest: Disclosures can be viewed at www.acponline.org/authors/icmje/ConflictOfInterestForms.do?msNum=M11-1964.

Reproducible Research Statement:Study protocol and data set: Not available. Statistical code: Available from Dr. Wegwarth (e-mail, wegwarth@mpib-berlin.mpg.de).

Requests for Single Reprints: Odette Wegwarth, PhD, Max Planck Institute for Human Development, Lentzeallee 94, 14195 Berlin, Germany; e-mail, wegwarth@mpib-berlin.mpg.de.

Current Author Addresses: Drs. Wegwarth, Gaissmaier, and Gigerenzer: Max Planck Institute for Human Development, Harding Center for Risk Literacy, Lentzeallee 94, 14195 Berlin, Germany.

Drs. Schwartz and Woloshin: Dartmouth Institute for Health Policy and Clinical Practice, 35 Centerra Parkway, Lebanon, NH 03766.

Author Contributions: Conception and design: O. Wegwarth, L.M. Schwartz, S. Woloshin, G. Gigerenzer.

Analysis and interpretation of the data: O. Wegwarth, L.M. Schwartz, S. Woloshin, W. Gaissmaier, G. Gigerenzer.

Drafting of the article: O. Wegwarth, L.M. Schwartz, S. Woloshin.

Critical revision of the article for important intellectual content: O. Wegwarth, L.M. Schwartz, S. Woloshin, G. Gigerenzer.

Final approval of the article: O. Wegwarth, L.M. Schwartz, S. Woloshin, W. Gaissmaier, G. Gigerenzer.

Provision of study materials or patients: O. Wegwarth.

Statistical expertise: O. Wegwarth, L.M. Schwartz, S. Woloshin, W. Gaissmaier, G. Gigerenzer.

Obtaining of funding: G. Gigerenzer.

Administrative, technical, or logistic support: O. Wegwarth, G. Gigerenzer.

Collection and assembly of data: O. Wegwarth.

Ann Intern Med. 2012;156(5):340-349. doi:10.7326/0003-4819-156-5-201203060-00005
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Reader Survey: Test your knowledge of cancer screening statistics

Background: Unlike reduced mortality rates, improved survival rates and increased early detection do not prove that cancer screening tests save lives. Nevertheless, these 2 statistics are often used to promote screening.

Objective: To learn whether primary care physicians understand which statistics provide evidence about whether screening saves lives.

Design: Parallel-group, randomized trial (randomization controlled for order effect only), conducted by Internet survey. (ClinicalTrials.gov registration number: NCT00981019)

Setting: National sample of U.S. primary care physicians from a research panel maintained by Harris Interactive (79% cooperation rate).

Participants: 297 physicians who practiced both inpatient and outpatient medicine were surveyed in 2010, and 115 physicians who practiced exclusively outpatient medicine were surveyed in 2011.

Intervention: Physicians received scenarios about the effect of 2 hypothetical screening tests: The effect was described as improved 5-year survival and increased early detection in one scenario and as decreased cancer mortality and increased incidence in the other.

Measurements: Physicians' recommendation of screening and perception of its benefit in the scenarios and general knowledge of screening statistics.

Results: Primary care physicians were more enthusiastic about the screening test supported by irrelevant evidence (5-year survival increased from 68% to 99%) than about the test supported by relevant evidence (cancer mortality reduced from 2 to 1.6 in 1000 persons). When presented with irrelevant evidence, 69% of physicians recommended the test, compared with 23% when presented with relevant evidence (P < 0.001). When asked general knowledge questions about screening statistics, many physicians did not distinguish between irrelevant and relevant screening evidence; 76% versus 81%, respectively, stated that each of these statistics proves that screening saves lives (P = 0.39). About one half (47%) of the physicians incorrectly said that finding more cases of cancer in screened as opposed to unscreened populations “proves that screening saves lives.”

Limitation: Physicians' recommendations for screening were based on hypothetical scenarios, not actual practice.

Conclusion: Most primary care physicians mistakenly interpreted improved survival and increased detection with screening as evidence that screening saves lives. Few correctly recognized that only reduced mortality in a randomized trial constitutes evidence of the benefit of screening.

Primary Funding Source: Harding Center for Risk Literacy, Max Planck Institute for Human Development.


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Figure 1.

Lead-time bias and overdiagnosis bias.

In lead-time bias, survival rates are inflated by earlier diagnosis even if mortality remains; in overdiagnosis bias, survival rates are inflated by the detection of nonprogressive cancer even if mortality remains unaltered. Figure reproduced from reference 2 with permission of the American Medical Association.

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Figure 3.

Physicians' understanding of which screening statistics provide evidence that screening saves lives.

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Figure 4.

Proportion of physicians who would recommend a screening test on the basis of survival versus mortality rates.

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