# T.C ATILIM UNIVERSITY MODES ADVANCED SYSTEM SIMULATION MODES 650

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T.C ATILIM UNIVERSITY MODES ADVANCED SYSTEM SIMULATION MODES 650

A COMPREHENSIVE REVIEW OF METHODS FOR SIMULATION OUTPUT ANALYSIS
Christos Alexopoulos School of Industrial and Systems Engineering Georgia Institute of Technology Atlanta, Georgia 30332–0205, U.S.A. represented by: Adel Agila

Introduction Simulation output analysis
Point estimator and confidence interval Variance estimation (σ2) confidence interval Independent and identically distributed (IID) Suppose X1,…Xm are iid

The Methods The Replication/Deletion Approach The Regenerative Method
The Batch Means Method nonoverlapping Batch Means(NBM) Overlapping Batch Means(OBM) Consistent Batch Means Estimation Methods The Standardized Time Series Method(STS) The Weighted Area Estimator Batched Area Estimators

types of simulations with regard to output analysis:
Finite-horizon simulations. In this case the simulation starts in a specific state and is run until some terminating event occurs. EX: Bank The output process is not expected to achieve any steady-state behavior. Steady-state simulations. The purpose of a steady-state simulation is the study of the long-run behavior of the system of interest. EX: Hospital

Finite-horizon simulations.
n output data are needed X1, X2,..., Xn are collected with the objective of estimating is the sample mean of the data. Xi can be : The transit time of unit i through a network , or the total time station i is busy during the ith hour. Then is an unbiased estimator for μ Xi are generally dependent random variables

Finite-horizon simulations.
Then, let be the sample variance of the data Then, the estimator a biased estimator of If Xi are positively correlated , To overcome the problem, run k independent replications

Replications Xij are the output data can be illustrated as:
and the replicate averages are are (IID)random var’s Here their means is unbiased estimator of μ and their sample variance is an unbiased estimator of Approximate 1-α cI for u 0<α<1, Tk-1,1- α/2 = upper critical point for t distribution,k-1 DOF

Stationary process :The process X = {Xi} is called stationary if the joint distribution of is independent of i for all indices j1,j2, . . ., jk and all k ≥ 1. weakly stationary process: If E(Xi) = µ, Var(Xi) ≡ σX2 < ∞ for all i, and the Cov (Xi ,Xi+j) is independent of i, then X is called WSP. σX2 the (asymptotic) variance parameter of X.

Stationary Process The stochastic process X is stationary for t1,…,tk, t∈ T, if Stationary time series with positive autocorrelation Stationary time series with negative autocorrelation Nonstationary time series with an upward trend

Stationary Process A discrete-time stationary process X = {Xi : i≥1} with mean µ and variance σX2 = Cov(Xi,Xi), Variance of the sample mean Then (the (asymptotic) variance parameter) For iid,

Stationary Process The expected value of the variance estimator is:
If Xi are independent, then is an unbiased estimator of If the autocorrelation is positive, then is biased low as an estimator of If the autocorrelation is negative, then is biased high as an estimator of

Stationary Process Cov(X, Y) = Σ ( Xi - X ) ( Yi - Y ) / N = Σ xiyi / N where N is the number of scores in each set of data X is the mean of the N scores in the first data set Xi is the ithe raw score in the first set of scores xi is the ith deviation score in the first set of scores Y is the mean of the N scores in the second data set Yi is the ithe raw score in the second set of scores yi is the ith deviation score in the second set of scores Cov(X, Y) is the covariance of corresponding scores in the two sets of data

Functional Central Limit Theorem (FCLT) Assumption.
Suppose the series is convergent, and σ2 >0. where Rj= Cov ( Xi ,Xi+j) , and σ2 : -the (asymptotic) variance parameter of X - equals As n→∞ ,we have the following convergent: t ≥ 0.

Assumption: Along the above equation 0<σ2 <∞. Imply
The variance of the sample mean in terms of the autocovariance function is Assumption: Along the above equation 0<σ2 <∞. Imply The paper focuses on methods for obtaining CIs for μ , which involve estimating σ2 . CI:1-α Confidence Interval

The Replication/Deletion Approach
K independent replications each of length l +n observations. Discard the first l observations from each run. Use the IID sample means Compute the point estimate If k is large ,compute the approximate 1-α cI for μ: Ex: Note

The Replication/Deletion Approach
For (l ,n, and k) (a) As l increased for fixed n, the “systematic” error in each Yi(l , n) due to the initial conditions decreased. (b) As n increased for fixed l , the systematic and sampling errors in Yi(l , n) decreased. (c) #of replications k cannot effect The Yi(l , n) k. (d) For fixed n, the CI is valid only if l / lnk → ∞ as k → ∞. l increase faster than lnk. Replication method (more )is expensive

The Regenerative Method
The basic concept underlying this approach is that for many systems a simulation run can be divided into a series of cycles such that the evolution of the system in a cycle is a probabilistic replica of the evolution in any other cycle. IID cycles The Method (we have) Random time indices 1≤T1<T2 <…. The portion (XTi+j,j) ≥ 0 has the same distribution for i . Then And the SS_mean Where E(Zi) < ∞ , E(Zi) ≠0

The Regenerative Method
To obtain estimates of the expected value of some random variable X Y1=62-53=9 Y3= =7 Z1=62-24=38 Y2=70-62=8

The Regenerative Method

The Regenerative Method
Disadvantages difficult to apply in prac­tice because the majority of simulations have either no regenerative points or very long cycle lengths.

The Batch Means Method nonoverlapping Batch Means(NBM)
To compute points and CI estimators for the mean µ. The method suppose the sample x1, x2,….xn . Divide the sample into k batches with m observations (n=km). Then, for i=1,2,……,k, the ith batch consists of the observations X(i+1)m+1 , X(i+1)m+2,….,Xim And the ith batch mean The NBM-based estimator of the mean is

Batch means method (Nonoverlapping batch mean)
Non-overlapping batch mean (NBM) m observations with batch mean Y1,m Batch 1 m observations with batch mean Yk,m Batch k

Nonoverlapping batch mean (NBM)
Suppose the batch means become uncorrelated as m  ∞ NBM estimator for σ2 Confidence Interval

Consistent Batch Means Estimation Methods
Alternative rules that yield strongly consistent estimators for The Assumption of Strong Approximation(ASA) Given a constant , and a finite random variable C such that, as n →∞ Where w(n) is a standard Brownian motion process. λ→1/ normal distribution and low correlation among Xi . λ→ the absence of one of the above .

The Assumption of Strong Approximation(ASA)
Theorem suppose ASA is hold, mn is batch sizes and kn is batch counts Such that , as n →∞ And for some finite integer q ≥ 1. Then, and Where N(0,1) is a standard normal random variable.

Overlapping Batch Means(OBM)
For a given m, this method uses all n-m+1 overlapping batches to estimate µ and the first batch X1,…….,Xm, the second batch X2,……….., Xm+1.etc The OBM estimator of µ is Where (batch mean) The OBM estimator of σ2 is Where k=n/m.

Overlapping Batch Mean (OBM)
Y1,m Y2,m OBM estimator for σ2

NBM vs. OBM Under mild conditions Variance of the estimators
Thus, both have similar bias Variance of the estimators Thus, the OBM method gives better (asymptotic)performance than NBM.

The Standardized Time Series Method(STS)
The estimator based on STS applied to batches. The method For the sample X1,X2,…..Xn. Define D0,n=0, and Di,n=Ŷi-Ŷn, for i=1,….,n; Scales the sequence Di,n by and the time by setting t=i/n. Then STS is If X satisfies a FCLT, then as n →∞

Standardized Time Series
Define the ‘centered’ partial sums of Xi as Central Limit Theorem Define the continuous time process Question: How does Tn(t) behave as n increases?

n=100

n=1000

n=10000

n=1,000,000 Unlike S, the S’ curve remains erratic even on larger time scales. However, the sample paths of S’ do converge to a limiting process given by the FCLT

The Weighted Area Estimator
To estimate σ2 We define the square of the weighted area under the standardized time series. its limiting functional is Where And satisfies the following If the above hold, then Where denotes equivalence in distribution, and Nor(0,1) denotes the standard normal random variable.

The Weighted Area Estimator
Under assumption FCLT, the continuous mapping theorem (CMT) implies A(f)=σ2Xv2 ,where Xv2 denotes chi-squared random variable with v degrees of freedom. And var[A(f)]=var[σ2Xv2 ]=2σ4.

Batched Area Estimators
Divide the run into contiguous, nonoverlapping batches Form an STS estimator from each batch. Take the average of the estimators. The STS from batch i ( i=1,2,….K then Where (i=1,…,k),&(j=1,….m) If Assumption FCLT holds,

Batched Area Estimators
Where (i) the [Zi: i = 1,……,k] are IID. standard normal random variables; (ii) the [Zi: i =1,…….,k] are independent of the Bs; and (iii) Bs denotes a standard Brownian bridge on [s,s+1], for The area estimator from batch i is

Batched Area Estimators
and the batched area estimator for σ2 is Since the Ti,m, i = 1,...,k, converge to independent Brownian bridges as m becomes large (with fixed k), we shall assume that the the Ai (f; m) are asymptotically independent as m →∞. Then ,we have Then, the variance of the batched area estimator : as m →∞.