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Exercise: Reactions to immunisation using different sized needles, 1

Question 1: The authors say that on the first day the rate of redness with the longer needle was
two thirds the rate with the smaller needle
(relative risk 0.66 (95% confidence interval 0.45 to 0.99), P=0.04).
What do they mean by 'relative risk 0.66' and is their interpretation correct?

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Suggested answer

Risk means the proportion of people with a given exposure to a risk factor
who will experience a given outcome.
The risk of redness for babies immunised with short needles is the proportion
of such babies whose parents report redness.

The relative risk of redness is the proportion
of babies immunised with long needles whose parents report redness
divided by the proportion
of babies immunised with short needles whose parents report redness.

As the relative risk is 0.66, the proportion
of babies immunised with long needles whose parents report redness is 0.66 times the proportion
of babies immunised with short needles whose parents report redness.
0.66 is approximately 2/3 (= 0.66666666666667).
Hence the statement that 'the rate of redness with the longer needle was
two thirds the rate with the smaller needle' is correct.

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