well said bedkobs and absolutely right !
YMF is asking all the wrong questions with his usual rhyming slang
Now suppose a thousand women are screened and it is expected that one has cervical cancer and the screening test has a false positive rate ( it dings positive when they dont have it ) of 5%.
then fifty have tested positive of which one has the disease
49 will have further testing and anxiety for no benefit
the rate of having the disease whcih was i/1000 or 0.1%
after the screen is now one in fifty or 2%
as you can see we are talking really small numbers
you can also put money on this so you can decided whether you want to screen or perhaps do more hip replacement £5000 a pop with 97% approval rate.
spose a smear costs £30 and further screening £100
one thousand women costs £30k to have a smear
and fifty positives will cost a further £ 5000
which is £35 000 - or seven hip replacements
and you will see that to save one life - in this example - is not the cost of a smear (£30) but more than a thousand times that ( 35k) - but it allows you to make the health care choices
Now YMF [ yeah yeah you often ask me why I write this stuff and I ofter ask myself that as well ] you may ask: " is there a number that if high tells me that a screening test is GOOD and if it is low tells me that it is not much good ? and the answer is YES the PPV ( positive predictive value ) and in our example above it is the figure 2%. This counts as low and tells us the screeninng in this instance isnt much good. You will also see that it is a mix of the prevalence ( 1/1000) and false pos rate.
well done if you have got this far
I have just looked up ce ca in 20-25 y age group
http://www.cancerresearchuk.org/health-professional/cervical-cancer-incidence-statistics#heading-One
and it is 2 per 100 000 - so to pick up 2 in 100 000 you would pay £3m.
I cant be arissed to go thro the whole calculation again.....
even I show signs of fatigue