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Table 6 Confidence intervals

From: SUPPORT Tools for evidence-informed health Policymaking (STP) 16: Using research evidence in balancing the pros and cons of policies

A confidence interval (CI) is the range around an estimate which conveys how precise the estimate is. The confidence interval is a guide that represents how sure it is possible to be about the quantity we are interested in (e.g. the effect of a policy option on an outcome of interest). The narrower the range between the upper and lower numbers of the confidence interval the more precise the estimate is and the more confident it is possible to be about the true value. The wider the range the less certain it is possible to be. The width or range of the confidence interval reflects the extent to which chance may be responsible for an observed estimate (wider intervals reflect the greater likelihood of chance being a factor). A 95% CI means that we can be 95% confident that the true size of an effect is between the lower and upper confidence limit. Conversely there is a 5% chance that the true effect is outside this range.
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