Sample sizes for cancer trials where Health Related Quality of Life is theprimary outcome

Health Related Quality of Life (HRQoL) instruments are increasingly importa nt in evaluating health care, especially in cancer trials. When planning a trial, one essential step is the calculation of a sample size, which will a llow a reasonable chance (power) of detecting a pre-specified difference (e ffect size) at a given level of statistical significance. It is almost mand atory to include this calculation in research protocols. Many researchers q uote means and standard deviations to determine effect sizes, and assume th e data will have a Normal distribution to calculate their required sample s ize. We have investigated the distribution of scores for two commonly used HRQoL instruments completed by lung cancer patients, and have established t hat scores do not have the Normal distribution form. We demonstrate that an assumption of Normality can lead to unrealistically sized studies. Our rec ommendation is to use a technique that is based on the fact that the HRQoL data are ordinal and makes minimal but realistic assumptions. (C) 2000 Canc er Research Campaign.