BMJ No 7106 Volume 315
Letters Saturday 23 August 1997
Intersalt data
Science demands data sharing
Editor,Research indicating the influence of dietary salt on blood pressure is convincing.(1) However, the size and scientific methodology of the Intersalt study give particular weight to its conclusions.(2) That the Salt Institute should put a counter case is not surprising(3) since the scientific debate takes place in a market economy.
Accepted scientific practice demands that results should be reproducible by independent scientists. Financial considerations mean that it is not realistic to reproduce data comparable to those of the Intersalt study, but the conflict between the Intersalt Cooperative Research Group and the Salt Institute focuses on the reproducibility and validity of the statistical analysis of the Intersalt data.
While it is reasonable for medical researchers to retain their data until they have published their primary analysis, the Intersalt Steering and Editorial Committee(4) claims the data as `the confidential property of local investigators' and so forces dissenters into a limited framework of scientific discussion.
Though I would not wish to support the analyses of Richard L Hanneman,(3) I find I have as many doubts about the statistical analysis of the Intersalt group and the commentaries on Hanneman's analysis.(3) Some of my qualms are:
 | the criss crossing of population trends mentioned by Hanneman, and described by Malcolm Law in his commentary as 'bizarre' and 'implausible,' is seen to occur if the trend lines are plotted | |
| the claim by J Stamler and colleagues that the use of the intercept is statistically invalid(3) is incorrect, and the biological argument used is spurious | |
| the expected effect of a difference of 100 mmol in 24 hour sodium excretion (for example, 70 v 170 mmol) over an age range of 30 years (for example, 25 v 55) is problematic, since the magnitude of the standard error of this measure depends critically on the end values of the intervals that are chosen | |
| has the Intersalt group taken into account the possible inhomogeneity of the variances about the regressions? Has the prediction theory been correctly based on the errors in regressor variables model? |
For the medical-scientific community to achieve a shared understanding it is necessary that confirmatory secondary analyses be carried out by other researchers. For these reasons, and the powerful ethical arguments made in Tony Delamothe's editorial,(4) the sharing of medical research data is long overdue. A good model is provided by the Economic and Social Research Council's data archive, which ensures use only by bona fide researchers and that the primary researchers are appropriately acknowledged.
Keith Rennolls Professor of applied statistics
School of Computing and Mathematical Sciences,
University of
Greenwich,
London SE18 9PF
References
1 MacGregor G A, Sever P S. Salt - overwhelming evidence but still no action: can a consensus be reached with the food industry? BMJ 1996;312:1287-9.
2 Elliott P, Stamler J, Nichols R, Dyer A R, Stamler R, Kesteloot H, et al for the Intersalt Cooperative Research Group. Intersalt revisited: further analysis of 24 hour sodium excretion and blood pressure within and across populations. BMJ 1996;312:1249-53.
3 Hanneman R L. Intersalt: hypertension rise with age revisited. [With commentaries by M Law and J Stamler et al.] BMJ 1996;312:1283-7.
4 Delamothe T. Whose data are they anyway? BMJ 1996;312:1241.
Home | Current issue | Past issues | Classified ads | Career Focus | Feedback
Collections | About this site | About the BMJ | BMA | Medline