Solution
= Solution
The line compares the two pooled accident rates, dividing the total number of accidents by total observation years in each period. It gives the unadjusted <incidence rate ratio>
$$
\boxed{\frac{15/18}{114/68}=0.497076,}
$$
corresponding to an estimated reduction of about $50.3\%$. Pooling does not adjust for differences between locations or for the unequal location composition of observation time before and after installation.