inference on phase-type models via mcmc1 both c1 and c2 work 2 c1 failed, c2 working 3 c1 working,...

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Inference on Phase-type Models via MCMC with application to networks of repairable redundant systems Louis JM Aslett and Simon P Wilson Trinity College Dublin 28 th June 2012

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Page 1: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Inference on Phase-type Models via MCMCwith application to networks of repairable redundant systems

Louis JM Aslett and Simon P Wilson

Trinity College Dublin

28th June 2012

Page 2: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Toy Example : Redundant Repairable Components

C1

C2

State Meaning

1 both C1 and C2 work2 C1 failed, C2 working3 C1 working, C2 failed4 system failed

∴ a general stochastic process, e.g.

1234

System Failed

Y (t)

t

Page 3: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Continuous-time Markov Chain Model

State Meaning

1 both C1 and C2 work2 C1 failed, C2 working3 C1 working, C2 failed4 system failed

C1 downC2 up

C1 upC2 down

C1 downC2 down

C1 upC2 up

!f !f

!f !f

!r !r

!u

=⇒ π =

100

,T =

−2λf λf λf 0λr −λr − λf 0 λfλr 0 −λr − λf λf0 0 0 0

Page 4: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Inferential Setting

Cano & Rios (2006) provide conjugate posterior calculations inthe context of analysing repairable systems when the stochasticprocess leading to absorption is observed.

DataFor each system failure time, one has:

• Starting state

• Length of time in each state

• Number of transitions between each state

• Ultimate system failure time

Reduced information scenario =⇒ Bladt et al. (2003) providea Bayesian MCMC algorithm, or Asmussen et al. (1996) providea frequentist EM algorithm.

Page 5: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Inferential Setting

Cano & Rios (2006) provide conjugate posterior calculations inthe context of analysing repairable systems when the stochasticprocess leading to absorption is observed.

DataFor each system failure time, one has:

• Starting state

• Length of time in each state

• Number of transitions between each state

• Ultimate system failure time

Reduced information scenario =⇒ Bladt et al. (2003) providea Bayesian MCMC algorithm, or Asmussen et al. (1996) providea frequentist EM algorithm.

Page 6: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Definition of Phase-type Distributions

An absorbing continuous time Markov chain is one in whichthere is a state that, once entered, is never left. That is, then+ 1 state intensity matrix can be written:

T =

(S s0 0

)

where S is n× n, s is n× 1 and 0 is 1× n, with

s = −Se

Then, a Phase-type distribution (PHT) is defined to be thedistribution of the time to entering the absorbing state.

Y ∼ PHT(π,S) =⇒

FY (y) = 1− πT exp{yS}e

fY (y) = πT exp{yS}s

Page 7: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Relating to the Toy Example

State Meaning

1 both C1 and C2 work2 C1 failed, C2 working3 C1 working, C2 failed4 system failed

C1 downC2 up

C1 upC2 down

C1 downC2 down

C1 upC2 up

!f !f

!f !f

!r !r

!u

=! T =

!""#

"2!f !f !f 0!r "!r " !f 0 !f

!r 0 "!r " !f !f

0 0 0 0

$%%&

!2!f !f !f 0!r !!r ! !f 0 !f

!r 0 !!r ! !f !f

0 0 0 0

S s

fY (y) = πT exp{yS}s FY (y) = 1− πT exp{yS}e

Page 8: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Bladt et al: Gibbs Sampling from Posterior

Strategy is a Gibbs MCMC algorithm which achieves the goalof simulating from

p(π,S |y)

by sampling fromp(π,S,paths · |y)

through the iterative process

p(!,S |paths ·,y)

p(paths · |!,S,y)

Page 9: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Bladt et al: Metropolis-Hastings Simulation of Process

In summary:• Can simulate chain from

p(path · |Yi ≥ yi)

trivially by rejectionsampling.

• A Metropolis-Hastingsacceptance ratio (ratio ofexit rates) exists sttruncating chain to time yi(at which point it absorbs)will be a draw from

p(path · |Yi = yi)

Metropolis-Hastings

Rejection Sampling

CTMC Samplingp(path · |!,S)

p(path · |!,S, Y ! y)

p(path · |!,S, Y = y)

Page 10: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Motivation for Modifications

1 Certain state transitions make no physical sense. (eg 2→ 3in earlier example)

2 When part of a larger system, it is highly likely there willbe censored observations.

3 Where there is no reason to believe distributionaldifferences between parameters, they should (in idealisedmodelling sense) be constrained to be equal. This is asmuch to assist with reducing parameter dimensionality.

4 Computation time!

Page 11: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Toy Example Results

100 uncensoredobservations simulatedfrom PHT with

S =

−3.6 1.8 1.89.5 −11.3 09.5 0 −11.3

=⇒ λf = 1.8, λr = 9.5

Parameter Value

Den

sity

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.5 1.0 1.5 2.0

Param

S12

S13

s2

s3

F

S12

S13

s2

s3

!f

Reliability less sensitive to λr(Daneshkhah & Bedford 2008)

Page 12: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Toy Example Results

100 uncensoredobservations simulatedfrom PHT with

S =

−3.6 1.8 1.89.5 −11.3 09.5 0 −11.3

=⇒ λf = 1.8, λr = 9.5

Parameter Value

Den

sity

0.0

0.1

0.2

0.3

0.4

8 9 10 11 12 13 14 15

Param

S21

S31

R!r

S21

S31

Reliability less sensitive to λr(Daneshkhah & Bedford 2008)

Page 13: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Toy Example Results

100 uncensoredobservations simulatedfrom PHT with

S =

−3.6 1.8 1.89.5 −11.3 09.5 0 −11.3

=⇒ λf = 1.8, λr = 9.5

Parameter Value

Den

sity

0

2

4

6

8

10

12

0.0 0.1 0.2 0.3 0.4 0.5

Param

s1

S23

S32

S23

S32

s1

Reliability less sensitive to λr(Daneshkhah & Bedford 2008)

Page 14: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

The Big Issue

Intractable computation time for many applications!

1 longer chains and MCMC jumps to states for whichobservations are far in the tails can stall rejectionsampling step of MH algorithm.

2 states from which absorption impossible – wastefulto resample whole chain because state at time yiunsuitable for truncation.

3 time for MH algorithm to reach stationarity cangrow rapidly.

Page 15: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

The Big Issue

Intractable computation time for many applications!

1 longer chains and MCMC jumps to states for whichobservations are far in the tails can stall rejectionsampling step of MH algorithm.

p(!,S |paths ·,y)

p(paths · |!,S,y)

P(Yi ! yi |!,S) = 10!6

2 states from which absorption impossible – wastefulto resample whole chain because state at time yiunsuitable for truncation.

3 time for MH algorithm to reach stationarity cangrow rapidly.

Page 16: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

The Big Issue

Intractable computation time for many applications!

1 longer chains and MCMC jumps to states for whichobservations are far in the tails can stall rejectionsampling step of MH algorithm.

E(RS iter.) = 106

95% CI = [25317, 3688877]

p(!,S |paths ·,y)

p(paths · |!,S,y)

P(Yi ! yi |!,S) = 10!6

2 states from which absorption impossible – wastefulto resample whole chain because state at time yiunsuitable for truncation.

3 time for MH algorithm to reach stationarity cangrow rapidly.

Page 17: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

The Big Issue

Intractable computation time for many applications!

1 longer chains and MCMC jumps to states for whichobservations are far in the tails can stall rejectionsampling step of MH algorithm.

2 states from which absorption impossible – wastefulto resample whole chain because state at time yiunsuitable for truncation.

1234

yi simulationinvalidtruncation

3 time for MH algorithm to reach stationarity cangrow rapidly.

Page 18: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

The Big Issue

Intractable computation time for many applications!

1 longer chains and MCMC jumps to states for whichobservations are far in the tails can stall rejectionsampling step of MH algorithm.

2 states from which absorption impossible – wastefulto resample whole chain because state at time yiunsuitable for truncation.

State Meaning P(state)

1 both PS working 0.99862 1 failed, 2 working 0.00073 1 working, 2 failed 0.0007

=⇒ E(MH iter) = 1429

95% CI = [36, 5267]

3 time for MH algorithm to reach stationarity cangrow rapidly.

Page 19: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

The Big Issue

Intractable computation time for many applications!

1 longer chains and MCMC jumps to states for whichobservations are far in the tails can stall rejectionsampling step of MH algorithm.

2 states from which absorption impossible – wastefulto resample whole chain because state at time yiunsuitable for truncation.

3 time for MH algorithm to reach stationarity cangrow rapidly.

0 100 200 300 400 500

0.0

0.2

0.4

Iterations

Tota

l Var

iatio

n D

ista

nce

Page 20: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Solution: “Exact Conditional Sampling”

Metropolis-Hastings

Rejection Sampling

CTMC Samplingp(path · |!,S)

p(path · |!,S, Y ! y)

p(path · |!,S, Y = y)

Page 21: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Solution: “Exact Conditional Sampling”

Metropolis-Hastings

Rejection Sampling

CTMC Samplingp(path · |!,S)

p(path · |!,S, Y ! y)

p(path · |!,S, Y = y)i) Starting state ~ discrete

ii) Advance time ~ Exponential

iii) Select next state ~ discrete

iv) If not absorbed, , loop to ii

! ! Exp("SY {t},Y {t})

t = t + !

Y {0} ! !

Y {t + !} ! {"SY {t},!Y {t}/SY {t},Y {t}}

Page 22: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Solution: “Exact Conditional Sampling”

Metropolis-Hastings

Rejection Sampling

CTMC Samplingp(path · |!,S)

p(path · |!,S, Y ! y)

p(path · |!,S, Y = y)

CTMC Samplingp(path · |!,S, Y = y)

e.g. Starting state mass function changes with conditioning:

P(Y {0} = i |!,S, Y = y) =eT

i exp{Sy}s !i

!T exp{Sy}s

P(Y {0} = i |!,S) = !i

Page 23: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Tail Depth Performance Improvement

Upper tail probability (10^-x)

log[

Tim

e (s

ecs)

]

10-3

10-2

10-1

100

101

102

2 3 4 5 6

MethodECS

MH

Page 24: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Overall Performance Improvement

This shows the new method keeping pace in ‘nice’ problems andsignificantly outperforming otherwise.

T =

(−3 1 1 11 −3 1 11 1 −3 10 0 0 0

)T =

( −2 0.01 1.99 01 −300 0 299

299 0 −300 10 0 0 0

)

No problems i-iii All problems i-iii

MH ECSt̄ 1.6 µs 7.2 µsst 104 µs 19 µs

MH ECS10.2 hours 0.016 secs9.4 hours 0.015 secs

2,300,000 × faster on average in hard problem

Page 25: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

Page 26: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

2

1

Page 27: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

1

2

1

2

Page 28: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

2

1

1

2

Page 29: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

2

1

1

2

Page 30: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

2

2

1

1

Page 31: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

1

2

2

1

Page 32: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

1

2

2

1

Page 33: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

1

1

2

1

Page 34: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

1

1

3

2

Page 35: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

2

1

3

1

Page 36: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Networks/Systems of Components

Quick overview now of current research focus: inference fornetworks/systems comprising nodes/components which may beof Phase-type.

?

?

?

?

T = t

Again, reduced information setting: overall network failuretime, or so-called ‘Masked system lifetime data’.

Page 37: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Direct Masked System Lifetime Inference

Even a very ‘simple’ setting quite hard to tackle directly.

X1 X2

X3

Xiiid∼ Weibull(scale = α, shape = β)

System lifetimes t = {t1, . . . , tn}

F̄T (t) = 1−(1− F̄X1(t)F̄X2(t))(1− F̄X3(t))

=⇒ L(α, β; t) =

m∏

i=1

t−1i β(ti/α)β exp

{−3(ti/α)β

}[2 exp

{(ti/α)β

}

+ exp{

2(ti/α)β}− 3]

∴ p(α, β | t) ∝ L(α, β; t)p(α, β) awkward.

Page 38: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

MCMC Solution (Independent Case)

Proposed solution in the tradition of Tanner & Wong (1987),since inference easy in the presence of (augmented) componentlifetimes.Thus, for Xi

iid∼ FX( · ;ψ) sample from the natural completion ofthe posterior distribution:

p(ψ,x1 · , . . . ,xn · | t)

by blocked Gibbs sampling using the conditional distributions:

p(x1 · , . . . ,xn · |ψ, t)p(ψ |x1 · , . . . ,xn · , t)

where xi · = {xi1, . . . , xim} are the m component failure timesfor the ith of n systems (xij = tj some j)

Page 39: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

p(ψ |x1 · , . . . ,xn · , �t) is now simple Bayesian inference forsystem lifetime distribution — well understood and inPhase-type case, above algorithm slots in here.

Problem shifted to sampling p(x1 · , . . . ,xn · |ψ, t)

Propose using Samaniego’s system signature sj = P(T = Xj:n)

e.g. p(xi1, . . . , xim |ψ, t) =m∑

j=1

{p(xi1, . . . , xim |ψ,Xj:n = ti)

×P(T = Xj:n |ψ, ti)}

where

P(T = Xj:n |ψ, ti) ∝ sj(m− 1

j − 1

)FX(ti)

j−1F̄X(ti)m−j−1

Page 40: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

p(ψ |x1 · , . . . ,xn · , �t) is now simple Bayesian inference forsystem lifetime distribution — well understood and inPhase-type case, above algorithm slots in here.

Problem shifted to sampling p(x1 · , . . . ,xn · |ψ, t)

Propose using Samaniego’s system signature sj = P(T = Xj:n)

e.g. p(xi1, . . . , xim |ψ, t) =

m∑

j=1

{p(xi1, . . . , xim |ψ,Xj:n = ti)

×P(T = Xj:n |ψ, ti)}

where

P(T = Xj:n |ψ, ti) ∝ sj(m− 1

j − 1

)FX(ti)

j−1F̄X(ti)m−j−1

Page 41: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Algorithm to sample p(x1 · , . . . ,xn · |ψ, t)

For each system i = 1, . . . , n:

1 Sample j ∈ {1, . . . ,m} from the discrete probabilitydistribution defined by the conditioned system signature,P(T = Xj:n |ψ, ti). This samples the order statisticindicating that the jth failure caused system failure.

2 Sample:• j − 1 values, xi1, . . . , xi(j−1), from FX |X<ti( · ;ψ), the

distribution of the component lifetime conditional on failurebefore ti

• m− j values, xi(j+1), . . . , xin, from FX |X>ti( · ;ψ), thedistribution of the component lifetime conditional on failureafter ti

and set xij = ti.

Each iteration provides xi · �

Page 42: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

All Systems of 4 Components, Exponential (λ = 0.4)

Network

Rat

e

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

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1 2 3 4 5 6 7 8 9 1011121314151617181920

Page 43: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Exchangeable Failure Rate Parameters

It is straight-forward to allow the more general setting ofexchangeable failure rate parameters between networks:

Xij�i� Ti

j = 1, . . . , n

i = 1, . . . ,m

However, exchangeability within network would break thesignature-based sampling of node failure times so this isprobably as general as this particular approach can go.

Page 44: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

All Systems of 4 Components, Ψ ∼ Gam(α = 9, β = 12)

Network

Pop

ulat

ion

Mea

n of

Rat

e D

istr

ibut

ion

4.0

4.5

5.0

5.5

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Page 45: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

All Systems of 4 Components, Ψ ∼ Gam(α = 9, β = 12)

Network

Pop

ulat

ion

Varia

nce

of R

ate

Dis

trib

utio

n

1

2

3

4

5

6

7

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Page 46: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Extend to Topological Inference

It is now possible to add topology detection to the inferentialprocess.

? ?

??

T = t

It is simple to sample from p(M|ψ, t) as an additional Gibbsupdate, moving between network topologies. With care,reversible jump between component numbers is also feasible.

Page 47: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

True Topology with Signature No. 12 (40 observation)

Network

Pos

terio

r P

roba

bilit

y

0.00

0.05

0.10

0.15

0.20

0.25

4 6 8 10 12 14 16 18

Page 48: Inference on Phase-type Models via MCMC1 both C1 and C2 work 2 C1 failed, C2 working 3 C1 working, C2 failed 4 system failed C1 down C2 up C1 up C2 down C1 down C2 down C1 up C2 up!

Intro Phase-type Distributions Bayesian Inference for PHT Computational Issues Network Inference

Asmussen, S., Nerman, O. & Olsson, M. (1996), ‘Fitting phase-typedistributions via the EM algorithm’, Scand. J. Statist.23(4), 419–441.

Bladt, M., Gonzalez, A. & Lauritzen, S. L. (2003), ‘The estimation ofphase-type related functionals using Markov chain Monte Carlomethods’, Scand. Actuar. J. 2003(4), 280–300.

Cano, J. & Rios, D. (2006), ‘Reliability forecasting in complexhardware/software systems’, Proceedings of the FirstInternational Conference on Availability, Reliability and Security(ARES’06) .

Daneshkhah, A. & Bedford, T. (2008), Sensitivity analysis of areliability system using gaussian processes, in T. Bedford, ed.,‘Advances in Mathematical Modeling for Reliability’, IOS Press,pp. 46–62.

Tanner, M. A. & Wong, W. H. (1987), ‘The calculation of posteriordistributions by data augmentation’, Journal of the AmericanStatistical Association 82(398), 528–540.