In reply to veteye:
' there is questioning going on at the moment about the value of screening for breast cancer.' I'm not an epidemiologist though I have studied it to Masters degree level.
IIRC screening for breast cancer was being trialled in the 80s wen Thatcher brought the trials to an end for political reasons - she needed to be seen to be supporting the NHS, even though the point of the trials was to measure whether they worked or not.
The problems with screening are threefold. 1) Do the number of false positives - i.e. people diagnosed wrongly with a disease then subjected to significant invasive procedures, which themselves cause mortality - justify the cost? 2) Do the number of false negatives - i.e. people who have the disease but are given the all clear, and therefore respond late when the disease becomes symptomatic, outweigh those who are successfully treated? And 3) (the real killer) How do you measure whether successful screening works or not?
If an asymptomatic patient is picked up by screening and treated, and survives for 5 years then you can say that that patient enjoyed 5 years of life thanks to screening. But if the same patient only presented 4 years later when the disease became symptomatic, then the stats would say they only survived a year after presenting, so that screening would 'obviously' have been advantageous ... except that they would lived the same length of time, and without screening with less stress and better quality of life because they didn't know...