AbstractAs caries prevalence in some countries declines, and panel sizes for discriminating anti‐caries clinical trials inevitably increase, the need for participant pre‐selection grows to avoid escalating cost and manpower problems. Hence, retrospective analyses of data were undertaken, from a 3‐yr double‐blind caries clinical trial where a linear fluoride dose‐response had been obtained. As a result, it was demonstrated that the eruption status of permanent second molars, in 3005 participants aged 11–12 yr at outset, when coupled with their baseline clinical DMFT values, provided a sound basis for panellist size prediction without loss of sensitivity. Thus, had only those subjects with DMFT>4, with al least three of their permanent second molars erupted been selected following clinical baseline examination, the same sensitivity would have been achieved with 52% fewer subjects. Furthermore, using these criteria, selection can be made on site, in real time, and a child informed whether its immediate participation in a trial