A SIMPLIFIED METHOD FOR THE ESTIMATION OF INDIVIDUAL RISK A. P. FRANKS 1 and T. MADDISON 2 1 ERM, Manchester, UK 2 Health and Safety Executive, Bootle, Merseyside, UK A simplified method for estimating individual risk is presented. The method described is a development of other semi-quantitative approaches such as the risk matrix or Layer of Protection Analysis (CCPS, 2001). The method provides a simplified means of obtaining a conservative estimate of the individual risk to members of defined popu- lation groups. It can also be used to identify those event outcomes contributing most to the risk for each of the population groups specified, and may be used as a screening tool to identify areas for further analysis. The method may be implemented within a spreadsheet. However, the effort involved in using the method increases rapidly as the numbers of event outcomes, population groups and hazardous material locations are increased. It is recommended that use of the method be considered when: † the number of event outcomes of interest is modest (50–100); † the hazardous materials on site are found at a few discrete locations (1–3); and † the number of population groups of interest is small (five or less). This paper and the work it describes were funded by the UK Health and Safety Executive. Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect ERM or HSE policy. Keywords: risk; analysis; methodology. THE RISK MATRIX The risk matrix is a well-known semi-quantitative risk assessment approach that has found widespread use amongst risk assessment practitioners. Although a risk matrix provides a useful, graphical portrayal of the risks presented by the system under study, difficulties arise when attempts are made to compare the risks as displayed on a risk matrix with individual risk criteria such as those published by the UK Health and Safety Executive (HSE) (HSE, 2001). This is because the matrix comprises a series of frequency-number of injury/fatality (f-n) pairs, whereas the HSE criteria are expressed in terms of individual risk of fatality. The different points on the matrix may also relate to a mixture of population groups. The method described in this paper seeks to address this problem by providing a semi-quantitative means of estimating individual risk, based on a development of the process used to gen- erate a risk matrix. DESCRIPTION OF THE METHOD The method is designed for use following the application of a hazard identification technique such as HAZOP, and review of the hazard identification study results to generate a list of events for analysis. The method then comprises the following steps: (1) Define probability and frequency categories for use in the study. (2) Define population groups of interest and their characteristics. (3) Define event outcomes of interest. (4) Estimate frequencies of event outcomes. (5) Estimate consequences of event outcomes. (6) Determine impacts of event outcomes at locations of interest. (7) Estimate individual risk. Each of these steps is described in more detail in sub- sequent sections. It should be noted that Steps 3–6 inclusive are essentially the same as the corresponding steps under- taken for the purposes of constructing a risk matrix.  Correspondence to: Dr A. P. Franks, ERM, Suite 8.01, 8 Exchange Quay, Manchester M5 3EJ, UK. E-mail: andrew.franks@erm.com 101 0957–5820/06/$30.00+0.00 # 2006 Crown Copyright www.icheme.org/journals Trans IChemE, Part B, March 2006 doi: 10.1205/psep.04287 Process Safety and Environmental Protection, 84(B2): 101–108