Intended for healthcare professionals

Rapid response to:

Views & Reviews Review of the Week

Dare to know: risk illiteracy and shared decision making

BMJ 2011; 342 doi: https://doi.org/10.1136/bmj.d2075 (Published 06 April 2011) Cite this as: BMJ 2011;342:d2075

Rapid Response:

Positive response

Maybe it's not difficult to work out.

A prevalence of 1% means that one women in a hundred has the disease.

If a hundred women are screened the one true case will test positive
and be picked up (90% of the time anyway: that's what sensitivity means).
So one positive.

The 99 women who don't have the disease have a 9% chance of testing
positive (that's what false positive means). So nine positives.

That's ten positive results (1+9) but only one of them is true.

The difficulty doctors have in understanding conditional probability
has been exacerbated by the lack of clarity and obfuscation of the
infomation leaflets produced by advocates of screening. [1,2,3]

dr.johndoherty@gmail.com

References

[1] Susan Mayor. NHS breast screening leaflet glosses over harms, say
critics. BMJ 2010;341:c7267

[2] Baum M, McCartney M, Thornton H. et al. Breast cancer screening
peril. Negative consequences of the breast screening programme . The Times
19th February 2009.
http://www.timesonline.co.uk/tol/comment/letters/article5761650.ece?prin...

[3] "Who evaluates public health programmes? A review of the NHS
Breast Screening Programme" KJ Jorgensen, PC Gotzsche J R Soc Med 2010:
103: 14-20.

Competing interests: No competing interests

09 May 2011
John Doherty
Retired
Vienna 1040