Intended for healthcare professionals


Absence of evidence is not evidence of absence

BMJ 2004; 328 doi: (Published 26 February 2004) Cite this as: BMJ 2004;328:476
  1. Phil Alderson (, associate director
  1. UK Cochrane Centre, Oxford OX2 7LG

    We need to report uncertain results and do it clearly

    The title of this editorial is not new. For example, it was used nearly a decade ago for an article in the BMJ's Statistics Notes series.1 Altman and Bland considered the dangers of misinterpreting differences that do not reach significance, criticising use of the term “negative” to describe studies that had not found statistically significant differences. Such studies may not have been large enough to exclude important differences. To leave the impression that they have proved that no effect or no difference exists is misleading.

    As an example, a randomised trial of behavioural and specific sexually transmitted infection interventions for reducing transmission of HIV-1 was published in the Lancet.2 The incidence rate ratios for the outcome of HIV-1 infection were 0.94 (95% confidence interval 0.60 to 1.45) and 1.00 (0.63 to 1.58) for two intervention groups compared with control. In …

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