The British Medical Journal has retracted an article that found UK households bought 10% less sugar in the form of soft drinks after the government started taxing the manufacturers on the sugar in their products.
The authors of the paper found an error in their analysis when following up on the work, and republished a corrected version – with less flashy results – in BMJ Open.
The original article, “Changes in soft drinks purchased by British households associated with the UK soft drinks industry levy: controlled interrupted time series analysis,” appeared in March 2021. It has been cited 84 times in the scientific literature, according to Clarivate’s Web of Science, as well as by media outlets and by policy documents for the UK government and World Health Organization.
In May 2022, Jean Adams, the paper’s corresponding author and a professor of dietary public health at the University of Cambridge, posted a “rapid response” to the online version of the paper alerting readers to the issue the authors had found:
In the process of conducting additional analyses exploring the impact of the Soft Drinks Industry Levy (SDIL) on purchasing of drinks and confectionary at around 2 years post implementation we have identified that an error was made in the analyses reported in this paper. We are working with the editorial teams at both BMJ to ensure a corrected version is published as soon as possible. In the meantime we urge caution in citing the results of this paper.
This month, a year and a half later, BMJ published a retraction, and also pulled an editorial that had run with the original publication.
The retraction notice explained:
The authors themselves identified the problem and alerted the journal and readers about inaccuracies arising from the way in which the data were prepared for analysis. Correcting the error changed the findings of the paper. The reduction in sugar intake per household was less than previously reported, and an increase in the volume of purchased drinks was found. The journal judged that the policy implications of the paper were affected by these changes and did not think that correction of the paper would be the appropriate course of action.
We thank the authors for their cooperation throughout this investigation, The BMJ’s statistical editor, and the three external experts who provided post-publication advice to the journal on this matter.
Adams provided a comment from the authors that went into further detail:
During preparation for the original analysis, a weighting variable was incorrectly calculated. This variable was also redundant to the analysis replicating a second weighting variable.
…
Most of the results in the paper are impacted by the correction – some by a negligible amount, others more substantively. Overall, the estimate of change in purchasing of sugar from all soft drinks combined at 1 year post implementation of the levy reduces from -30g (or -10%) per household per week to -8g (or -3%) per household per week. Alongside, the estimate of change in volume of soft drinks purchases increased from no change to a 189ml (or 3%) increase per household per week.
Adams told us the authors “have made substantial attempts not just to correct the record through working with the BMJ and BMJ Open, but also to let relevant colleagues know.” She said they had alerted all corresponding authors of papers that cited the original, hosted a briefing for non-profits focused on diet and obesity in the UK, and notified UK government officials working on topics related to the tax. The authors also updated their funders and scientific steering group “throughout the process,” Adams said.
In response to our question why it had taken a year and a half for the retraction and republished paper to appear, Adams said:
The timeline was mostly driven by the BMJ. There were at least two extra peer review processes – one to help inform their decision over whether this could be a correction or a retraction with republication; and then another of the corrected paper submitted to BMJ Open.
A BMJ spokesperson told us:
We have no other comment to add other than this was a complex case that required a re-analysis.
We asked Adams if she had any advice for other researchers who find themselves in a similar situation. She said:
I think the key thing I’ve learnt (remembered) is to remain humble and that everyone makes mistakes. Making mistakes is to be expected. What matters is that when we find them, we own up to them and fix them. I found this guidance from ICMJE helpful, particularly “honest errors are a part of science.”
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