Imagine the situation: you’re in a clinic and in comes a 7 year old child with a belly ache. The ache has been there, on and off for 3 years. Investigations have been undertaken for at least 2 years, in two different centres, and have included blood, stool, radiological and invasive procedures. No clear diagnosis has ever emerged. The parents insist that there is a cause — just that you haven’t found it yet. Sound familiar? Most clinicians will inwardly groan at the thought, and may turn to struggle to understand why the parents keep fixed on the hope of a diagnosis, instead of just accepting there isn’t one and moving on. Yet – can you also recall a situation where a clinician has done something similar? Lept to a rare or interesting explanation for a symptom cluster and despite negative tests, has held to a ‘niggling doubt’ that the real answer is underneath? It’s a natural phenomenon that in making decisions, we start off somewhere, and only very reluctantly move away from the anchoring point we started at. Other ‘diagnostic heuristics’ (shortcuts of the mind) that we use when making diagnoses include availability (if you’ve just had a Kawasaki Disease, then every febrile child has it creep into their differential) and representativeness (the power of stereotypes – ‘children like that have this’) which, if examined carefully, don’t always stand up to any evidence that can be produced. In practicing evidence-based child health, we need to be aware of how we make pre-test estimates, and accept that tests will make us move our position (even if it ruins the grand round case we were already preparing in our heads).