Update: regular commenter Mark L has helpfully identified they reason behind the apparent anomaly in the statistics that motivated me to write this post. I had misinterpreted one of the statistics. While this takes the mystery out of the numbers, it does highlight how tricky it can be to get to grips with the statistics of medical tests. I have edited this post to correct that misinterpretation. I decided not to use a the strikethough editing approach popular among bloggers as the content can be confusing enough already!
Despite the fact that more banks have been failing (Bradford & Bingley, Wachovia, Hypo Real Estate, Fortis,…), in this post I will continue to stay away from the subject of the financial markets and will instead look at some mathematics, trying not to lose too many readers in the process.
Recently I was contemplating results from an “NT-plus” test, which combines ultrasound measurements of the nuchal translucency with maternal blood-tests to provide a screening test for various chromosomal abnormalities, particularly Down Syndrome. Tests of this type abound with statistics and the mathematician in me could not resist crunching the numbers a little to get a better understanding of the test.