A. When People Mistake no Evidence of Effect for Evidence of no Effect.
Imagine that you have malignant melanoma on your forearm. You can select wide margin excision or a narrow margin. The latter is obviously less disfiguring.
Results from six RCTs (n=4,233) have been consolidated in a meta-analysis. In keeping with individual trials and with previous meta-analyses, the result is null for numerous outcomes. However, the point estimates all favour wider margins and the confidence limits are close to the (arbitrary) 0.5% significance level. For example, the hazard ratio for overall survival favouring wide margins is 1.09 (0.98-1.22). The authors state that the study shows “a 33% probability that [overall survival] is more than 10% worse” when a narrow margin excision is used. It should be added that this assumes an uninformative prior. If the prior probability estimate favoured better survival with wider excision margins, then the evidence in favour of a wider margin excision is stronger still. Moreover, the authors quote results showing that patients do not trade even small survival gains for improved cosmetic outcome. Despite loose statistical language (conflating the probability of survival given the data with the probability of the data if there was no difference in outcome), the authors have done science and practice a great service. This paper should be quoted in the context of surgical treatment of cancer, not just melanoma excision. For example, is sentinel biopsy really preferred to axillary dissection in breast cancer surgery?
— Richard Lilford, CLAHRC WM Director
- Wheatley K, Wilson JS, Gaunt P, Marsden JR. Surgical excision margins in primary cutaneous melanoma: A meta-analysis and Bayesian probability evaluation. Cancer Treat Rev. 2015. [ePub].