# Fault Tree Quantification > Use when you must quantify a fault-tree top event from its minimal cut sets and basic-event probabilities: compute the rare-event approximation (sum of cut-set probabilities), the Esary-Proschan min-cut upper bound, and the exact inclusion-exclusion top probability when 2^n stays tractable; truncate the cut sets below an analyst probability threshold and keep the retained probability share; rank the cut sets by per-cut-set probability share to flag dominant failure combinations. Produces the quantified top-event probability, its bound chain, and the truncated cut-set mass picture, the predicted-probability input the SSA closure comparison consumes. Trigger: fault-tree quantification, rare-event approximation, min-cut upper bound, cut-set truncation, cut-set probability share, top-event probability. - Skill: `ashfordeou/fault-tree-quantification` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/fault-tree-quantification` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/fault-tree-quantification/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/fault-tree-quantification --- # Fault Tree Quantification (systems-engineering-safety/arp4761a/fault-tree-quantification) Use when you must quantify the top-event probability of a fault tree from its minimal cut sets and basic-event probabilities. This leaf computes the rare-event approximation, the Esary-Proschan min-cut upper bound, the exact inclusion-exclusion top probability when the cut-set count keeps 2^n tractable, the probability-threshold truncation of the cut sets with the retained probability share, and the per-cut-set probability-share ranking that flags the dominant failure combinations. It is the missing producer in the FTA-to-SSA chain: it pairs with systems-engineering-safety/arp4761a/fta-fmea, which derives the minimal cut sets and checks their sanity against a supplied top probability, and with systems-engineering-safety/arp4761a/ssa-closure, which consumes the quantified top number as the predicted probability per flight hour. All logic is deterministic, offline, pure stdlib. ## Domain quick reference - Inputs: cut_sets, a list of sets of basic-event names (one set per minimal cut set), and probs, a dict mapping every basic-event name to a probability strictly inside (0, 1). Basic events are independent; a cut set occurs when every basic event in it fails, at probability equal to the product of its member probabilities. - Rare-event approximation: Q_re = sum of the cut-set probabilities, the first-order union approximation, exact only for one cut set and always at or above the true union probability. - Esary-Proschan min-cut upper bound: Q_ep = 1 - product over the cut sets of (1 - cut-set probability), an upper bound on the exact top probability of a coherent tree with independent basic events. In real arithmetic Q_ep <= Q_re always; the float survival product near 1.0 has an absolute rounding floor around 1e-16, so compare with Q_ep <= Q_re + 1e-15. - Exact inclusion-exclusion: Q_ie = sum over the non-empty subsets of the cut sets of (-1)^(k+1) times the product of the basic-event probabilities over the union of the k selected cuts (independent events make the intersection product exactly that union product). The 2^n - 1 term count is capped at EXACT_IE_MAX_CUT_SETS = 12 (4095 terms). - Bound chain on a tree with shared events: Q_ie < Q_ep <= Q_re, because the inclusion-exclusion pair terms subtract the overlap mass the rare-event sum counts twice. - Truncation: retained_probability_share = sum of the retained cut-set probabilities divided by the sum of all cut-set probabilities; drop is strict below the analyst threshold (probability equal to the threshold is kept), threshold in (0, 1]. - Ranking: per-cut-set probability share = cut-set probability divided by the sum of all cut-set probabilities, sorted descending with a stable input-order tie-break; shares sum to 1.0. - ARP4761A frames the quantitative safety assessment context; the relations above are standard engineering methodology, summary-only. ## Workflow 1. Gather the analysis inputs from the fta-fmea leaf: the minimal cut sets (list of event-name sets) and the basic-event probabilities per flight hour (dict), and confirm every event in every cut set has a probability strictly inside (0, 1); an empty list, an empty cut-set entry, a missing or out-of-range probability is rejected with ValueError before any pass runs. 2. Run the rare-event approximation pass with rare_event_probability: Q_re is the sum of the cut-set probabilities, the first-order top-event estimate. 3. Run the Esary-Proschan min-cut upper bound pass with min_cut_upper_bound: Q_ep = 1 - product(1 - q_i) brackets the exact top probability from above. 4. Run the exact inclusion-exclusion pass with exact_top_probability when the cut-set count is at most EXACT_IE_MAX_CUT_SETS = 12; at 13 or more cut sets the guard raises ValueError and the step 2 and step 3 approximations bracket the top number instead. 5. Truncate the cut-set list with truncate_cut_sets at the analyst probability threshold, drop strictly below the threshold, and record the retained_probability_share of the rare-event mass kept. 6. Rank the cut sets with rank_cut_sets by per-cut-set probability share, descending with input-order tie-break, and flag the dominant failure combinations where risk reduction must go first. 7. Assemble the SSA predicted-probability input: the quantified top-event probability, the bound chain Q_ie < Q_ep <= Q_re that brackets it, and the truncated cut-set mass picture, for the ssa-closure condition comparison. 8. Confirm determinism and input rejection with the contract test: python3 scripts/test_fault_tree_quantification.py. ## Worked example Elevator actuation supply loss, dual channel: two hydraulic pumps, a shared reservoir, two servo-valve channels and two flight-control computers feed one actuation function. The fta-fmea leaf derived five minimal cut sets of the top event; several cuts share events (P-A in cuts 1 and 2, P-B in cuts 1 and 3), so the overlap correction is non-trivial and exact IE is tractable at n = 5. Basic-event probabilities per flight hour: C-A 1.0e-05, C-B 1.0e-05, P-A 1.0e-04, P-B 1.0e-04, RSV 2.0e-05, V-A 5.0e-05, V-B 5.0e-05 (C = flight control computer, P = pump, RSV = shared reservoir, V = servo valve). - Cut sets in input order: {P-A, P-B} both pumps lost (q_cut 1.0e-08), {P-A, RSV} pump A and reservoir lost (2.0e-09), {P-B, RSV} pump B and reservoir lost (2.0e-09), {V-A, V-B} both valve channels jammed (2.5e-09), {C-A, C-B} both computers lost (1.0e-10). - Q_re (rare-event approximation) = 1.66e-08. - Q_ep (Esary-Proschan min-cut upper bound) = 1.660000015224483e-08. - Q_ie (exact inclusion-exclusion) = 1.6599599963351046e-08. - Bound chain: Q_ie < Q_ep <= Q_re with Q_ep - Q_ie = 4.001889e-13 and Q_re - Q_ie = 4.000366e-13, the overlap mass the pair terms subtract; the rare-event approximation overestimates the exact top probability on this overlapping-cut tree. The quantified top number 1.66e-08 per flight hour is the predicted probability the SSA closure condition comparison consumes. - Cut-set truncation at threshold 1e-9 keeps the 4 cuts above 1e-10 ({C-A, C-B} drops) with retained_probability_share 0.9939759036144578; at threshold 6e-9 only the dominant {P-A, P-B} cut at 1e-8 stays, share 0.6024096385542169: the both-pumps cut alone carries 60.24 percent of the failure mass. - Per-cut-set probability-share ranking (descending, input-order tie-break): #1 {P-A, P-B} share 0.6024096385542169, #2 {V-A, V-B} 0.15060240963855423, #3 {P-A, RSV} 0.12048192771084339, #4 {P-B, RSV} 0.12048192771084339 (tied pair keeps input order), #5 {C-A, C-B} 0.00602409638554217; shares sum to 1.0. Pump independence or a second independent supply is the highest-leverage fix, ahead of computer redundancy, which holds only 0.6 percent of the mass. - Exact-pass guard: at 13 cut sets, above EXACT_IE_MAX_CUT_SETS = 12, exact_top_probability raises ValueError; at the acceptance boundary, 12 single-event disjoint cuts at 1e-6 give Q_ie = 1.1999934000219944e-05 against the closed form 1 - (1 - 1e-6)^12 = 1.1999934000583856e-05, a relative gap of 3.03e-11 from alternating-sum rounding at 4095 terms. ## Verification - Confirm rare_event_probability on the worked tree returns 1.66e-08 within 1e-20, min_cut_upper_bound returns 1.660000015224483e-08 within 1e-20, and exact_top_probability returns 1.6599599963351046e-08 within 1e-20. - Confirm the bound chain: Q_ep - Q_ie = 4.001889e-13 within 1e-18, Q_ep <= Q_re + 1e-15, Q_re - Q_ie = 4.000366e-13 within 1e-18. - Confirm a single cut set {P-A} at 2.0e-4 returns 2.0e-4 from all three top formulas within 1e-12 relative, and disjoint single-event cuts {P-A} at 2e-4, {P-B} at 3e-4 give Q_re = 5.0e-4 with Q_ep = Q_ie = the exact union a + b - a*b = 4.9994e-4. - Confirm truncation at 1e-9 keeps the 4 cuts above 1e-10 in input order with share 0.9939759036144578, at 6e-9 keeps only {P-A, P-B} with share 0.6024096385542169, a threshold equal to a cut probability (2e-9) keeps that cut, threshold 1.0 drops everything (share 0.0), and threshold 1e-12 drops nothing (share 1.0). - Confirm the ranking order and shares match the tabulated worked example and the shares sum to 1.0 within 1e-12. - Confirm exact_top_probability raises ValueError at 13 cut sets with a message containing "max 12" and matches 1 - (1 - 1e-6)^12 within 1e-9 relative at 12 single-event cuts. - Confirm ValueError rejection of non-physical inputs: empty cut_sets, an empty cut-set entry, an event with no probability entry, a probability of 1.5 and of 0.0, and a threshold of 1.5 and of 0.0. - Run the contract test offline: python3 scripts/test_fault_tree_quantification.py (33 tests, deterministic, exit 0). ## Related leaves - systems-engineering-safety/arp4761a/fta-fmea: derives the minimal cut sets from AND/OR gate structures and sanity-checks cut-set probabilities; the producer of this leaf's two inputs. - systems-engineering-safety/arp4761a/fault-tree-importance-measures: per-event Birnbaum, Fussell-Vesely, RAW and RRW ranking on top of the quantified tree, where exact inclusion-exclusion is the small-n re-evaluation engine behind each measure. - systems-engineering-safety/arp4761a/fault-tree-uncertainty-analysis: lognormal band around the quantified top probability this leaf produces. - systems-engineering-safety/arp4761a/ssa-closure: consumes the quantified top-event probability as the predicted probability per flight hour in the condition comparison. - systems-engineering-safety/arp4761a/event-tree-analysis: the forward dual that enumerates branch paths from an initiating event frequency instead of unioning cut events into a top number. - systems-engineering-safety/arp4761a/failure-rate-estimation: derives the basic-event probabilities from demonstration data before this leaf quantifies the tree. ## Pitfalls - Attempting exact inclusion-exclusion past the guard: the 2^n - 1 term count doubles with every cut set, so above EXACT_IE_MAX_CUT_SETS = 12 the module raises ValueError; the report should fall back to the rare-event approximation and the min-cut upper bound as the bracket. - Reading the rare-event approximation as exact: on trees with shared events across cuts Q_re overestimates the true top probability (Q_ie < Q_re by 4.000366e-13 on the worked tree), so the bound chain, not the bare sum, is the honest top-number statement. - Treating Q_ep <= Q_re as a float-guaranteed comparison: the Esary-Proschan bound evaluates a survival product near 1.0 whose absolute rounding floor is around 1e-16, so the contract asserts the ordering with a 1e-15 tolerance. - Misreading the truncation boundary: drop is strict below the analyst threshold, so a cut set whose probability equals the threshold is kept, and the threshold itself must lie in (0, 1]. - Re-deriving cut sets or ranking events here: gate traversal and minimal-cut-set derivation stay in the fta-fmea leaf and per-event importance ranking in fault-tree-importance-measures; this leaf only unions the cut events it is given into a top number. - Quoting per-cut-set shares without their sum check: the ranking shares must sum to 1.0, and the tied cuts keep input order, so a tie in the ranking is not a dominance statement. ## Behavior contract (gate 3) Run the deterministic contract test (stdlib unittest, offline): python3 scripts/test_fault_tree_quantification.py The test covers the worked tree anchors (Q_re 1.66e-08, Q_ep 1.660000015224483e-08, Q_ie 1.6599599963351046e-08, each within the spec magnitude bounds), the bound chain with its two overlap gaps, the single cut set and disjoint-pair union identities, probability-threshold truncation with the retained share, per-cut-set probability-share ranking with input-order tie-break, the 12-cut-set exact inclusion-exclusion guard and its closed-form boundary, determinism, exact dict keys, and ValueError rejection of empty cut sets, empty cut-set entries, missing event probabilities, out-of-range probabilities and out-of-range thresholds. All 33 tests pass offline in under 20 seconds. ## Compliance - Standards referenced, not reproduced: ARP4761A is proprietary (SAE); name + paraphrase only, summary-only per standards-map.yaml. The rare-event, Esary-Proschan and inclusion-exclusion relations are standard engineering methodology. - compliance: STANDARDS-REF, gated: false. Inputs (minimal cut sets, basic-event probabilities) come from the sibling fta-fmea leaf; this leaf adds the top-number production only.