where exp is the base of the natural logarithm raised to the
quantity in the brackets, and A, B, C, and D represent the numerical entries into
the summary format in Table 9–4. This confidence interval
is approximate because it is based on a computational short-cut
to estimating the variance of the natural logarithm of the OR. For relatively large sample sizes,
this approximation yields confidence bounds that are quite close
to the exact values, which are much more difficult to calculate.