Exercise: Reactions to immunisation using different sized needles, 3

Question 3: What conclusion should we draw about swelling?

Suggested answer

The authors report that the rate of swelling with the longer needle was initially about a third that with the smaller needle (relative risk 0.39 (0.23 to 0.67), P=0.0002).

From the confidence interval, we can conclude that in the wider population of babies the relative risk is between 0.23 and 0.67. From P value, the difference between long and short needles is highly significant. There is very strong evidence that the relative risk in the population is not equal to 1.0. Hence there is very good evidence that there is less swelling with the long needles than with the short needles.

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