Critical appraisal of published research: introductory guidelines.

Appendix: derivation of sensitivity formula N=number of women having triple test; p=prevalence of Down's syndrome; p*N=total number of fetuses with Down's syndrome; DR=detection rate; FPR=false positive rate; p*N*DR*AR=number of women with true positive results who have amniocentesis; (1-p)*N*FPR*AR= number of women with false positive results who have amniocentesis; p*N*DR*AR=number having an abortion (all positive amniocentesis results); TT=cost of triple test per person; Am=cost of amniocentesis; Ab=cost of abortion; AR= amniocentesis acceptance rate.