use (depending on the used ground rules). A failure mode may given a Probability Ranking with a defined number of levels. For a piece part FMEA, quantitative probability may be calculated from the results of a reliability prediction analysis and the failure mode ratios from a failure mode distribution catalog, such as RAC FMD-97. This method allows a quantitative FTA to use the FMEA results to verify that undesired events meet acceptable levels of risk. Severity (S) Determine the Severity for the worst-case scenario adverse end effect (state). It is convenient to write these effects down in terms of what the user might see or experience in terms of functional failures. Examples of these end effects are: full loss of function x, degraded performance, functions in reversed mode, too late functioning, erratic functioning, etc. Each end effect is given a Severity number (S) from, say, I (no effect) to V (catastrophic), based on cost and/or loss of life or quality of life. These numbers prioritize the failure modes (together with probability and detectability). Below a typical classification is given. Other classifications are possible. See also hazard analysis. Detection (D) The means or method by which a failure is detected, isolated by operator and/or maintainer and the time it may take. This is important for maintainability control (availability of the system) and it is especially important for multiple failure scenarios. This may involve dormant failure modes (e.g. No direct system effect, while a redundant system / item automatically takes over or when the failure only is problematic during specific mission or system states) or latent failures (e.g. deterioration failure mechanisms, like a metal growing crack, but not a critical length). It should be made clear how the failure mode or cause can be discovered by an operator under normal system operation or if it can be discovered by the maintenance crew by some diagnostic action or automatic built in system test. A dormancy and/or latency period may be entered. Dormancy or latency period The average time that a failure mode may be undetected may be entered if known. For example: Seconds, auto detected by maintenance computer 8 hours, detected by turn-around inspection 2 months, detected by scheduled maintenance block X 2 years, detected by overhaul task x Indication If the undetected failure allows the system to remain in a safe / working state, a second failure situation should be explored to