Renal physicians in Sheffield want to examine the use of 3 doses of cinacalcet per week instead of the typical 7 doses for the treatment of secondary hyperparathyroidism in patients with end-stage renal disease on maintenance dialysis therapy. The daily dose was not deemed cost-effective in a NICE technology appraisal, but at 3/7th the cost, three doses may well be. When designing the trial we used PenTAG’s model to assess the probability that the three dose regime would be cost-effetive given different hypothesised efficacies and samples sizes (which defined the standard errors around efficacy). From a baseline result of £61,806 within the technology appraisal, a sample size of 30 per arm in the proposed trial and with 80% efficacy of the daily dose produces a 65% chance of treatment being cost-effective. A four-fold increase in sample size increases this to 68%, suggesting that a much larger trial may not be worthwhile.