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An approximate 95% confidence interval (CI) around the point estimate of the risk ratio (RR) can be calculated using the following formula:

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where exp is the base of the natural logarithm raised to the quantity within the brackets and A, B, C, and D represent the numerical entries in the summary format in Table 8–6, and VAR(lnRR) is the estimated variance of the natural logarithm of the RR. This confidence interval is approximate because it is based on a computational short-cut for estimating the variance of the natural logarithm of the RR. For relatively large samples, this approximation yields confidence limits that are quite close to the exact values, which are much more difficult to calculate.

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The 95% CI for the 10-minute Apgar score (0–3 versus 4–6) relationship to infant mortality described in Chapter 8: Cohort Studies is

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