Ancient Egyptian medical papyri (1550BC) emphasised diagnosis by examination as the cornerstone of the decision to treat or not to treat an ailment. Today the information about the likelihood of diagnosis (and related prognosis) that interventions purport to treat comes from a combination of tests as shown in Figure 1. In addition to easy-to-obtain history (risk factors and symptoms) and examination (signs), current testing includes sophisticated investigations (haematology, biochemistry, radiology, microbiology, etc), which can also be invasive and expensive. Obstetricians routinely incorporate many test results into decision-making concerning management of women at risk of preterm birth, but studies to examine the ways in which even a small number of these tests alter the benefit and harm of treatments are rare.