People are often unsure of the best method for estimating the detection rates and false positive rates in Down’s syndrome screening.
There are two main methods, the study specific method and the general method. The general method is the standard and preferred method because it provides more stable estimates of screening performance than the study specific method, which is subject to random sampling error. Also, while the study specific method is specific to the age distribution of the women in that particular study, the general method is not, an advantage that allows comparisons between studies.
For a detailed description of the two methods, see the article on the Wolfson Institute of Preventive Medicine website.