Journal of Public Health | Vol. 40, No. 2, p. 312 Correspondence Commentary on Piumatti et al. 2018 a car as beneﬁt, and reverse movement as harm, this is a claim that putting on the brakes is producing harm. This is a well-designed study with interesting results. Even if one did accept the bizarre argument that ‘harm’ Unfortunately, these results provide no justiﬁcation for the starts when the gradient starts increasing, there are two rea- headline conclusions drawn by the authors, which are in dir- sons why there is no foundation for any claim that the data ect contradiction to their own analysis. identiﬁes a threshold of 10 g/day. First, the authors claim in the Results section that � Daily alcohol consumption is plotted on a log-scale ‘Cognitive performance declined as alcohol consumption (although the fact that the model is ﬁtted on a log-scale increased beyond 10 g/day (Fig. 1)’. This is not true. provides no reason for this). If the graph were plotted on Looking at Fig. 1 clearly reveals that reaction times continue a standard linear scale, the curve would become convex, to improve beyond 10 g/day until reaching a minimum at with no straight lines, and so there would be no point of around 16 g/day, the current UK guidelines. inﬂexion at all, and the positive change in gradient would Second, in the discussion they claim that ‘Cognitive per- start from the lowest possible alcohol level. Therefore, formance improved as alcohol consumption increased up to the whole idea of a point of inﬂexion is a product of their 10 g/day and then deteriorated as alcohol consumption arbitrary choice of a logarithmic scale. increased beyond 10 g/day’. Again, this is in direct contra- � The claimed point-of-inﬂexion of 10 g/day was chosen as diction to their Fig. 1. such by the authors, as it is a pre-selected ‘knot’ in the They then go on to say ‘Our ﬁndings suggest that to pre- restricted cubic spline on log-consumption, where a serve cognitive performance 10 g/day is a more appropriate straight line is guaranteed to turn into a curve. upper limit.’ And yet their Fig. 1 clearly shows reaction times at this threshold are slower than at the current government It is, to put it very mildly indeed, deeply regrettable that such guidelines of 16 g/day. clearly unjustiﬁed conclusions have become part of the scientiﬁc These statements are repeated in the Abstract: ‘Cognitive literature, and have already been quoted uncritically by journalists. function declined as alcohol use increased beyond 10 g/ day….Consuming more than one UK standard unit of alco- David Spiegelhalter hol per day is detrimental to cognitive performance.’, none Centre for Mathematical Sciences, University of of which can be justiﬁed from their own analysis. Cambridge, Cambridge, UK I understand that the authors might argue that ‘harm’ starts at 10 g/day, since at this point the straight line of Fig. Address correspondence to David Spiegelhalter, 1 turns into a curve—technically, the gradient with respect E-mail: email@example.com to the logarithm of daily consumption starts increasing. This doi: 10.1093/pubmed/fdy038 is an extraordinary idea. If we identify forward movement of Advance Access Publication March 1, 2018 312 © The Author(s) 2018. Published by Oxford University Press on behalf of Faculty of Public Health. All rights reserved. For permissions, please e-mail: firstname.lastname@example.org Downloaded from https://academic.oup.com/jpubhealth/article-abstract/40/2/312/4915977 by Ed 'DeepDyve' Gillespie user on 24 July 2018
Journal of Public Health – Oxford University Press
Published: Mar 1, 2018
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