General factors affecting the choice of effect size to detect

For a definitive phase III trial, it maybe important to consider the potential public health impact of a treatment. Important factors to consider include:

♦ How common is the disease?

♦ How widely applicable is the treatment?

♦ What level of toxicity is expected?

In common diseases, simple treatments that are widely applicable with few side effects (such as aspirin after myocardial infarction) need not demonstrate large survival benefits before being accepted. Even in cancer, improving survival by 5 per cent in common tumours such as lung, colon and prostate cancer would save or prolong many thousands of lives. For example, there are 20,000 deaths from colorectal cancer in the UK every year; a 5 per cent survival improvement may save or prolong 1000 lives a year. However, the following points need to be born in mind:

♦ Before a large-scale public health trial can be conducted, a lot of people will need to be persuaded that the treatment is safe, and of potential benefit - this can usually only be done through smaller trials.

♦ More complicated, toxic, specialist treatments will have to demonstrate that any benefit in terms of survival is large enough to outweigh the increased toxicity and perhaps decreased quality of life - so the trial may not need to be extremely large.

In other words, large, moderate and small trials all have a place and will continue to be conducted. Freedman [7] discusses this issue further.

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