System events: unions or intersections of events

This example illustrates system events, which are defined as unions or intersections of other events. We will show how to estimate their probability both with Monte-Carlo sampling (using the class ProbabilitySimulationAlgorithm) and with a first order approximation (using the class SystemFORM).

Intersection

The event defined as the intersection of several events is realized when all sub-events occurs:

Union

The event defined as the union of several events is realized when at least one sub-event occurs:

from __future__ import print_function
import openturns as ot
import openturns.viewer as viewer
from matplotlib import pylab as plt
ot.Log.Show(ot.Log.NONE)

For system events, we always have to use the same root cause.

dim = 2
distribution = ot.Normal(dim)
X = ot.RandomVector(distribution)

Define some basic events E1, E2 and E3.

f1 = ot.SymbolicFunction(['x0', 'x1'], ['x0'])
f2 = ot.SymbolicFunction(['x0', 'x1'], ['x1'])
f3 = ot.SymbolicFunction(['x0', 'x1'], ['x0+x1'])

Y1 = ot.CompositeRandomVector(f1, X)
Y2 = ot.CompositeRandomVector(f2, X)
Y3 = ot.CompositeRandomVector(f3, X)

e1 = ot.ThresholdEvent(Y1, ot.Less(), 0.0)  # E1 <=> x0<0
e2 = ot.ThresholdEvent(Y2, ot.Greater(), 0.0) # E2 <=> x1>0
e3 = ot.ThresholdEvent(Y3, ot.Greater(), 0.0) # E3 <=> x0+x1>0

Define the intersection E3=E1 AND E2.

e4 = ot.IntersectionEvent([e1, e2])

Approximate probability of that event: .

e4.getSample(10000).computeMean()

[0.253]

Define the union E4=E1 OR E2.

e5 = ot.UnionEvent([e1, e2])

Approximate probability of that event: .

e5.getSample(10000).computeMean()

[0.7472]

It supports recursion: define E7=E1 OR (E1 AND E3).

e7 = ot.UnionEvent([e1, ot.IntersectionEvent([e2, e3])])
print(e3.isComposite())

Out:

True

With Monte-Carlo algorithm

Of course, we can use simulation algorithms.

experiment = ot.MonteCarloExperiment()
algo = ot.ProbabilitySimulationAlgorithm(e7, experiment)
algo.setMaximumOuterSampling(2500)
algo.setBlockSize(4)
algo.setMaximumCoefficientOfVariation(-1.0)
algo.run()
result = algo.getResult()
result.getProbabilityEstimate()

Out:

0.7479999999999998

SystemFORM

SystemFORM is an approximation method suitable for system events.

The event must be in its disjunctive normal form (union of intersections, or a single intersection).

root cause

dim = 5
mean = [200.0] * dim
mean[-1] = 60
mean[-2] = 60
sigma = [30.0] * dim
sigma[-1] = 15.0
R = ot.CorrelationMatrix(dim)
for i in range(dim):
    for j in range(i):
        R[i, j] = 0.5
dist = ot.Normal(mean, sigma, R)

leaf events

X = ot.RandomVector(dist)
inputs = ['M1', 'M2', 'M3', 'M4', 'M5']
e0 = ot.ThresholdEvent(ot.CompositeRandomVector(ot.SymbolicFunction(inputs, ['M1-M2+M4']), X), ot.Less(), 0.0)
e1 = ot.ThresholdEvent(ot.CompositeRandomVector(ot.SymbolicFunction(inputs, ['M2+2*M3-M4']), X), ot.Less(), 0.0)
e2 = ot.ThresholdEvent(ot.CompositeRandomVector(ot.SymbolicFunction(inputs, ['2*M3-2*M4-M5']), X), ot.Less(), 0.0)
e3 = ot.ThresholdEvent(ot.CompositeRandomVector(ot.SymbolicFunction(inputs, ['-(M1+M2+M4+M5-5*10.0)']), X), ot.Less(), 0.0)
e4 = ot.ThresholdEvent(ot.CompositeRandomVector(ot.SymbolicFunction(inputs, ['-(M2+2*M3+M4-5*40.0)']), X), ot.Less(), 0.0)

system event in DNF form (union of intersections)

event = ot.UnionEvent([ot.IntersectionEvent([e0, e3, e4]), ot.IntersectionEvent([e2, e3, e4])])

compute probability with basic sampling

event.getSample(10000).computeMean()

[0.0828]

Run systemFORM.

solver = ot.AbdoRackwitz()
solver.setMaximumIterationNumber(1000)
solver.setMaximumAbsoluteError(1.0e-3)
solver.setMaximumRelativeError(1.0e-3)
solver.setMaximumResidualError(1.0e-3)
solver.setMaximumConstraintError(1.0e-3)

algo = ot.SystemFORM(solver, event, mean)
algo.run()
result = algo.getResult()
result.getEventProbability()

Out:

0.07883551213326333