Determining the size of difference to detect in a specific trial

Sometimes the targeted difference to detect can be guided by previous trials or meta-analysis. Where such data are not available, it is common to make decisions based on round-table discussions. However, it may often be useful to supplement this informal process with a more formal one, especially, for example, where one is considering a new treatment, or one for which there maybe a difficult balance between benefits and 'costs.' Here, a more structured way of determining the appropriate difference may be useful. We have found formal questionnaires, completed by clinicians involved in the disease in question, to be a very useful means of addressing this issue.

The questionnaires described below are based on those developed by Freedman and Spiegelhalter [8]. They comprise two sections, the first identifying the expected differences in survival, the second attempting to elicit the clinically important differences as well as the factors which affect these values. In describing these questionnaires, the CHART bronchus trial [9] is used as an example. This was a randomized trial comparing conventional radiotherapy with Continuous, Hyperfractionated, Accelerated RadioTherapy (CHART) in the treatment of inoperable non-small cell lung cancer. Conventional radiotherapy was delivered in thirty fractions of 2 Gy, given once a day, Monday to Friday only, over six weeks. CHART was given to a total dose of 54 Gy, given in thirty-six fractions of 1.5 Gy, with three fractions given at 8-hour intervals for twelve consecutive days including weekends. This approach would, it was hoped, improve survival by minimizing the opportunity for tumour repopulation between fractions, and also reduce late morbidity. However, the timings make CHART difficult and expensive to organize and acute morbidity may be more severe.

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