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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