"Actually you would not expect it to increase the Downs rate at all - the null hypothesis is that there are X% Downs babies born and Y% identified through amniocentesis (where X-Y is z% the percentage of parents choosing termination)"
No, see, you're misunderstanding and making the exact same mistake by assuming that the nurses sat down and crunched the spreadsheets, which they obviously did not, and implicitly conditioning. Yes, if you slice the multivariable data just the right way to extract the conditional %, the increase would be the same. Screening more women won't make the conditional percentage go up.
However, if you are just a nurse, observing # of women coming in, and # of confirmed Down babies coming out... Screening more women will mechanically make the # of confirmed Down babies go up. And that's what the rate is, it's a count.
No, see, you're misunderstanding and making the exact same mistake by assuming that the nurses sat down and crunched the spreadsheets, which they obviously did not, and implicitly conditioning. Yes, if you slice the multivariable data just the right way to extract the conditional %, the increase would be the same. Screening more women won't make the conditional percentage go up.
However, if you are just a nurse, observing # of women coming in, and # of confirmed Down babies coming out... Screening more women will mechanically make the # of confirmed Down babies go up. And that's what the rate is, it's a count.