TABLE OF CONTENTS As noted in Chapter 1, risks can be expressed in absolute terms, for example, number of fatalities per mile year for permanent residents within one-half mile of pipeline Also common is the use of relative risk measures, whereby hazards are prioritized such that the examiner can distinguish which aspects of the facilities pose more risk than others. The former is a frequency-based measure that estimates the probability of a specific type of failure consequence. The latter is a comparative measure of current risks, in terms of both failure likelihood and consequence.
A criticism of the relative scale is its inability to compare risks from dissimilar systems pipelines versus highway transportation, for example and its inability to provide direct failure predictions. The absolute scale often fails in relying heavily on historical data, particularly for rare events that are extremely difficult to quantify, and on the unwieldy numbers that often generate a negative reaction from the public. The absolute scale also often implies a precision that is usually not available to any risk assessment method. So, the absolute scale offers the benefit of comparability with other types of risks, whereas the relative scale offers the advantage of ease of use and customization to the specific risk being studied.
Note that the two scales are not mutually exclusive. A relative risk ranking is converted into an absolute scale by equating previous accident histories with their respective relative risk values. This conversion is discussed in section IV on page 298. Absolute risk...
