2. Systems Engineering
2.6 Decision Analysis Tools
For decisions where no single clear analytical metric exists, the following approaches allow the use of subjective rankings between criteria to create numerical weightings [Akin]:
- Decision Matrices (aka Pugh Method) incorporate evaluation criteria and the relative importance of each criterion to narrow a list of options into one choice. The American Society for Quality offers a procedure, example, and considerations in developing a decision matrix [ASQ]. Mogan et al. created a Pugh Matrix highlighting various mission trade-offs for a Lunar Extraction for Extraterrestrial Prospecting mission:
- Analytic Hierarchy Process (AHP), created by Saaty, is a method of ranking used to derive ratio scales from both discrete and continuous paired comparisons. The original paper talks through the process through a couple of examples in a lengthy format for which Akin gives a simple example in slides 56-62. Guerra and Christian walk through an incredibly thorough example of AHP that motivates AHP use, the basic steps, and the evaluation metrics around the 2007 Mars Design Reference Mission Mars Surface Power System.
Prioritization matrix. Image courtesy of NASA.
- Quality Function Deployment (QFD) “was developed by the Japanese in 1972 to improve quality and lower costs in industrial and business-related fields, by assuring all of a company’s operational decisions are driven by customer needs. It uses a set of matrices to relate customer’s wants and needs with project specifications and requirements” [Dodd]. ASQ provides a procedure in how to develop the initial house of quality matrix and subsequent waterfall relationship of QFD matrices. Zapata at Kennedy Space Center used a QFD “to add definition and insight to the development of long term Highly Reusable Space Transportation (HRST)”.
- Six Sigma (6σ) is” a set of techniques and tools for process improvement. It was introduced by American engineer Bill Smith while working at Motorola in 1986” [Wikipedia]. “Six Sigma quality” is a term generally used to indicate a process is well controlled (within process limits ±3s from the centerline in a control chart, and requirements/tolerance limits ±6s from the centerline)” [ASQ].
- Six Sigma is more of a best practices methodology at an organizational level to transform businesses than an individual decision-making tool. The phases of business transformation include defining, measuring, analyzing, improving, and controlling.
- Key Six Sigma techniques include brainstorming, root cause analysis, the voice of the customer, the 5 S system, Kaizen (continuous improvement), Benchmarking, Poka-yoke (mistake proofing), and value stream mapping [Kumar]. NASA Glenn Research Center has generated its own root cause analysis tool to facilitate the analysis of anomalies, close calls, and accidents and also identify the appropriate corrective actions to prevent a recurrence.
- Six Sigma tools are statistical and data-driven: Cause and Effect Analysis, Flow Chart, Pareto Chart, Histogram, Check Sheet, Scatter Plot, Control Chart
