Risk on The Radar

Combine likelihood, consequence and detectability with a radar chart

Traditional risk assessments use two dimensions of analysis: the likelihood that a threat or hazard event will happen, and the consequence if the event does happen.1 Using graded number scales—increasing values for rising likelihood or consequence—risk analyses can be presented as plots of consequence vs. likelihood.

Contemporary risk assessments include a third dimension—detectability, which is the degree of difficulty of sensing an event through monitoring for process alerts or warnings.2,3 A detectability number scale at its low end means timely detection is certain; the high end means timely detection is nearly impossible.

Likelihood (L), consequence (C) and detectability (D) are multiplied together into a risk priority number (RPN) as:

RPN (LC) = L × C or RPN (LCD) = L × C × D.

The two RPNs are clearly distinguished on a radar chart.

Example from pipeline safety

Online Table 1 summarizes risk analyses applied to natural gas pipeline incidents in the United States during 2016.4 Likelihood values reflect relative frequencies of incidents, whereas consequence values increase from slow leaks to catastrophic explosions.

Online Table 1

Low detectability values denote excellent alerting from in-line inspection technologies, whereas high values indicate poor alerting, including ill-advised practices that evade or defeat monitoring. Such detectability-based distinction explains why leak/rupture detection has renewed emphasis in pipeline risk analysis.

Traditional consequence vs. likelihood (Online Figure 1) does not provide differentiation by detectability as does the RPN radar chart using a logarithmic axis (Figure 1). For example, natural force damage (such as from severe floods) might not be detected if physical access to the pipeline is impeded (RPN (LCD/LC) = 32/8) and rates a higher risk priority than material/weld/equipment failure, in which monitoring is more immediate (RPN (LCD/LC) = 18/9). The radar chart improves risk prioritization by detectability using RPN (LCD).

Online Figure 1

Figure 1


  1. American National Standards Institute (ANSI) and American Society of Safety Engineers (ASSE), ANSI/ASSE Z690.2-2011—Risk Management Principles and Guidelines—National Adoption of ISO 31000:2009, Jan. 11, 2011.
  2. R. Dan Reid, “Standards Outlook: FMEA—Something Old, Something New,” Quality Progress, May 2005, pp. 90-93.
  3. Jim Breneman, Dan Burrows and Mark Durivage, “In Focus,” Quality Progress, October 2017, pp. 18-24.
  4. “DIMP Implementation: What Gets Measured Gets Done,” National Association of Pipeline Safety Representatives and US DOT PHMSA Office of Pipeline Safety, October 2017, https://primis.phmsa.dot.gov/dimp/docs/WRGC_DIMP_Update_08.29.2017.pdf.

James L. Gooding is managing director of Geoclime, LLC, in Seabrook TX, which provides quality and risk consulting for the energy, water, science and engineering industries. He also has worked in the natural gas and electric-power industries. Gooding is a senior member of ASQ and an ASQ-certified manager of quality/organizational excellence.

--Bart Ellingsen, 03-10-2019

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