E-DRUG: How to read a paper
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Dear E-druggers,
Summary points from the British Medical Journal (BMJ) series on
How to read a paper follows. Full article can be accessed via the
Internet (http://www.bmj.com). Thanks.
Syed Rizwanuddin Ahmad
Email: srahmad@essential.org
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BMJ No 7105 Volume 315
Education and debate Saturday 16 August 1997
How to read a paper
Statistics for the non-statistician. II:
"Significant" relations and their pitfalls
Trisha Greenhalgh
This is the fifth in a series of 10 articles introducing non-experts
to finding medical articles and assessing their value. This article
continues the checklist of questions that will help you to appraise
the statistical validity of a paper.
Correlation, regression, and causation
For many non-statisticians, the terms "correlation" and
"regression" are synonymous, and refer vaguely to a mental image
of a scatter graph with dots sprinkled messily along a diagonal
line sprouting from the intercept of the axes. You would be right
in assuming that if two things are not correlated, it will be
meaningless to attempt a regression. But regression and correlation
are both precise statistical terms which serve quite different
functions.
Summary points
An association between two variables is likely to be causal if it
is strong, consistent, specific, plausible, follows a logical time
sequence, and shows a dose-response gradient
A P value of 0.05 means that this result would have arisen by chance
on less than one occasion in 20
The confidence interval around a result in a clinical trial indicates
the limits within which the "real" difference between the treatments
is likely to lie, and hence the strength of the inference that can be
drawn from the result
A statistically significant result may not be clinically significant.
The results of intervention trials should be expressed in terms of
the likely benefit an individual could expect (for example, the
absolute risk reduction)
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