# Appendix B: Method for Determining the Confidence Interval Around the Risk Ratio

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

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(ln*RR*) 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.

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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