<qa>  How many patients are required to develop reliable prediction models?The aim will be to compare the predictive ability of both models to ascertain how prognostic the relative decrease in exercise capacity is, after adjusting for baseline exercise. To develop reliable prediction models approximately 252 patients are required. This is using the Rule of 10 and assumes that the one year mortality rate is 20 conservative estimate and that models contain 5 factors e.g. age, gender, centre, location of tumour, laparoscopic versus open. 