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है”ह”ह

IS 15038 (2011): Reliability Growth – Statistical Test andEstimation Methods [LITD 2: Reliability of Electronic andElectrical Components and Equipment]

L___

RELIABILITYAND

September 2001

‘/

IS 15038:2001IEC 61164 (1995)

WR%?Wi’m

Yih-mw$.

~ W%Pi q-d

fadvl m

Indian Standard

GROWTH — STATISTICAL TESTESTIMATION METHODS

ICS 03.120.01; 03.120.30; 21.020

/-”-

Q BIS 2001

BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

Price Group 10

III

‘1i 1,

__—

Reliability of Electronic and Electrical Components and Equipment Sectional Committee, LTD 03

NATIONAL FOREWORD

This Indian Standard which is identical with IEC 61164 (1995) ‘Reliability growth — Statistical test andestimation methods’ issued by the International Electrotechnical Commission (IEC), was adopted by the Bureauof Indian Standards on the recommendation of Reliability of Electronic and Electrical Components andEquipment Sectional Committee and approval of the Electronics and Telecommunication Division Council.

In the adopted standard, certain conventions are, however, not identical to those used in Indian Standards. Attentionis particularly drawn to the following:

a) Wherever the words ‘International Standard’ appear referring to this standard, they should be read as‘Indian Standard’.

b) Comma (,) has been used as a decimal marker while in Indian Standards, the current practice is to use apoint (.) as the decimal marker.

Only English language text has been retained while adopting this International Standard.

CROSS REFERENCES

In this adopted standard, reference appears to the following International Standard for which Indian Standardalso exists. The corresponding Indian Standard which is to be substituted in its place is listed below along withits degree of equivalence for the edition indicated:

International Standard

IEC 50(19 I) :1990 InternationalElectrotechnical Vocabulary (IEV)— Chapter 191: Dependabilityand quality of service

Corresponding Indian Degree ofStandard Equivalence

IS 1885 (Part 39):1999 Electrotechnical Not Equivalentvocabulary: Part 39 Reliability ofelectronic and electrical items (secondrevision)

The technical committee responsible for the preparation of this standard has reviewed the provisions of thefollowing International Standards and has decided that they are acceptable for use in conjunction with thisstandard :

IEC 605-1:1978

IEC 605-4:1986

IEC 605-6:1986

IEC 1014:1989

Equipment reliability testing — Part 1: General requirements

Equipment reliability testing — Part 4: Procedures for determining point estimates andconfidence limits from equipment reliability determination tests

Equipment reliability testing — Part 6: Tests for the validation of a constant failure rateassumption

Programmed for reliability growth

/ l’!

-

. . —-

1S 15038:2001IEC 61164(1995)

Indian Standard

RELIABILITY GROWTH — STATISTICAL TESTAND ESTIMATION METHODS

1 Scope

This International Standard gives models and numerical methods for reliability growth assessmentsbased on failure data from a single system which were generated in a reliability improvementprogramme. These procedures deal with growth, estimation, confidence intervals for system reliabilityand goodness-of-fit tests.

2 Normative references

The following normative documents contain provisions which, through reference in this text, constituteprovisions of this International Standard. At the time of publication, the editions indicated were valid.All normative documents are subject to revision, and parties to agreements based on this InternationalStandard are encouraged to investigate the possibility of applying the most recent editions of thenormative documents listed below. Members of IEC and 1S0 maintain registers of currently validInternational Standards.

IEC 50(191): 1990, International Electrotechnical Vocabulary (IEV) - Chapter 191: Dependability andquality of service

IEC 605-1:1978, Equipment reliability testing - Part 1: General requirements

IEC 605-4: 1986, Equipment reliability testing - Part 4: Procedures for determining point estimatesand confidence limits from equipment reliability determination tests

IEC 605-6: 1986, Equipment reliability testing - Part 6: Tests for the validity of a constant failure rateassumption

IEC 1014:1989, Programmed for reliability growth

3 Definitions

For the purposes of this standard the terms and definitions of IEC 50(191) and IEC 1014 &ply,together with the following additional terms and definitions:

3.1 delayed modification: A corrective modification which is incorporated into the system at theend of a test.

NOTE - A delayed modification is not incorporated during the test. .

..—.,. -’-

3,2 improvement effectiveness factor: The fraction by which the intensity of a systematic faiIure isreduced by means of corrective modification.

IS 15038:2001

IEC 61164(1995)

3.3 type I test: A test which is terminated at a predetermined time or test with data availablethrough a time which does not correspond to a failure.

NOTE - Type I test is sometimes called time terminated test.

3.4 type II test: A reliability growth test which is terminated upon the accumulation of a specifiednumber of failures, or test with data available through a time which corresponds to a failure.

NOTE – Type 11test is sometimes called failure terminated test.

4 Symbols

For the purpose of this international standard, the following symbols apply:

Ki

M

N

Ni

N(T)

E[N(T)]

f(i–l); f(i)

T

~

TN

F

x;(v)z

‘Y

‘P

:(T)

E)(T)

(3P

scale and shape parameters for the power law model

critical value for hypothesis test

number of intervals for grouped data analysis

mean and individual improvement effectiveness factors

number of distinct types of category B failures observed

general purpose indices

number of category A failures

number of category B failures

number of i-th type category B failures observed; KB = ~ Kii=l

parameter of the Crarm%-vonMises test (statistical)

number of relevant failures

number of relevant failures in i-th interval

accumulated number of failures up to test time T

expected accumulated number of failures up to test time T

endpoints of i-th interval of test time for grouped data analysis

current accumulated relevant test time

accumulated relevant test time at the i-th failure

total accumulated relevant test times for type II test

total accumulated relevant test times for type I test

y fractile of the %2distribution with v degrees of freedom

general symbol for failure intensity

y fractile of the standard normal distribution

projected failure intensity

current failure intensity at time T

current instantaneous mean time between failures

projected mean time between failures

_... _

-

1.

. .

.-

1,

2

/ i

IS 15038:2001IEC 61164(1995)

5 The power law model

. +

.-. —

The statistical procedures for the power law reliability growth model use the original relevant failureand time data from the test. Except in the projection technique (see 7.6), the model is applied to thecomplete set of relevant failures (as in IEC 1014, figure 2, characteristic (3)) without subdivision intocategories.

The basic equations for the power law model are given in this clause. Background information on themodel is given in annex B.

The expected accumulated number of failures up to test time T is given by:

E[N(T)]=kTP, with L>O, ~>0, T>O

where

k is the scale parameter

~ is the shape parameter (a function of the general effectiveness of the improvements; Oc ~ e 1,corresponds to reliability growth; ~ = 1 corresponds to no reliability growth; P >1 corresponds tonegative reliability growth).

The current failure intensity after T h of testing is given by:

Z(T) =+ E[N(T)] = l~T&*, with T> O

Thus, parameters A and ~ both affect the failure intensity achieved in a given time. The equationrepresents in effect the slope of a tangent to the N(T) vs. T characteristic at time T as shown inIEC 1014, figure 6.

The current mean time between failures after T h of testing is given by:

@T).~Z(T)

Methods are given in 7.1 and 7.2 for maximum likelihood estimation of the parameters L and-#.Subclause 7.3 gives goodness-of-fit tests for the model, and 7.4 and 7.5 discuss confidence intervalprocedures. An extension of the model for reliability growth projections is given in 7.6.

The model has the following characteristic features:

it is simple to evaluate;

when the parameters have been estimated from past programmed it is aplanning future programmed employing similar conditions of testing andeffectiveness (see clause 5, and IEC 1014, clause 6);

convenient tool forequal improvement

--

k

it gives the unrealistic indications that z(T) = w at T = O and”that growth can be unending, that is

z(T) tends to zero as T tends to infinity; however, these limitations do not generally affect itspractical use;

it is relatively slow and insensitive in indicating growth immediately after a correctivemodification, and so may give a low (that is, pessimistic) estimate of the final e(T), unlessprojection is used (see 7.6);

3

/

.

IS 15038:2001IEC 61164(1995)

the normal evaluation method assumes the observed times to be exact times of failure, but analternative approach is possible for groups of failures within a known time period (see 7.2.2).

—.—

6 Use of the model in planning reliability improvement programmed

As inputs to the procedure described in 6.3 of IEC 1014, two quantities have to be predicted by meansof reliability growth models:

the accumulated relevant test time in hours expected to be necessary to meet the aims of theprogramme;

the number of relevant failures expected to occur during this time period.

The accumulated relevant test time is then converted to calendar time from the planned test time perweek or month, making allowance for the predicted total downtime (see below) and othercontingencies, and the number of relevant failures is increased by judgment to include non-relevantfailures and used to predict total downtime.

The inputs to the model for these calculations will be the assumed parameters for the model, as alreadyestimated from one or more previous programmed, and judged to be valid for the future application bysimilarity of the test items, test environment, management procedures and other significant influences.

7 Statistical test and estimation procedures

7.1 Overview

The procedures in 7.2 utilize system failure data during a test programme to estimate the progress ofreliability growth, and to estimate, in particular, the final system reliability at the end of the test. Thereliability growth which is assessed is the result of corrective modifications incorporated into thesystem during test. The procedures discussed in 7.2.1 assume that the accumulated test time to eachrelevant failure is known. Subclause 7.2.2 addresses the situation where achual failure times are notknown and failures are grouped in intervals of test time.

Type I tests, which are concluded at F, which is not a failure time, and type II tests, which areconcluded at failure time TN. use slightly different formulae, as indicated in 7.2.1 .

An appropriate goodness-of-fit test, as described in 7.3, shall be performed after the growth testprocedures of 7.2.1 and 7.2.2.

Subclause 7.6 addresses the situation where the corrective modifications are incorporated into thesystem at the end of the test as delayed modifications. The projection technique estimates the systemreliability resulting from these corrective modifications.

7.2 Growth tests and parameter estimation

7.2.1 Case 1 - Time data for every relevant failure

This method applies only where the time of failure has been logged for every failure.

4

1

Step 1:

Step 2:

Step 3:

.

IS 15038:2001—...—

IEC 61164(1995)

.+exclude non-relevant failures by reference to 7.1 of IEC 1014, and/or other appropriatedocumentation.

assemble into a data set thewhich each relevant failuretest.

Calculate the test statistic

accumulated relevant test times (as defined in 9.5 of IEC 605-1) atoccurred. For type I tests, note also the time of termination of the

or

y~-(N-1)$2

u = ‘=’

r

N-1row m

TN y

where.

N is the total number of relevant failures;

P’ is the total accumulated relevant test times for type I test;

TN is the total accumulated relevant test times for type 11test;

Ti is the accumulated relevant test time at the i-th failure.

Under the hypothesis of zero growth, that is, the failure times follow a homogeneous Poisson process,the statistic U is approximately distributed as a standard normal random variable with mean O andstandard deviation 1. The statistic U can be used to test if there is evidence of reliability growth,positive or negative, independent of the reliability growth model.

A two-sided test for positive or negative growth at the a significance ‘level has critical values

U1+12 and - U1_a12 , where U,+,2 is the (1-a/2). 100-th fractile of the standard normal distribution.

If u< -uI_@2 Ou u> UI-0J2

then there is evidence of positive or negative reliability growth, respectively, and the analysis iscontinued with Step 4.

If, however, -%cz/2 < fJ < %cl/2

then there is not evidence of positive or negative reliability growth at the a significance level and thegrowth analysis is terminated. In this case, the hypothesis of exponential times. between successivefailures (or a homogeneous Poisson process) is accepted at the a significance level. The critical values

‘1-a12 and - u1_a12correspond to a one-sided test for positive or negative growth, respectively, at the

cx/2 significance level.

At the 0,20 significance level, the critical values for a two-sided test are 1,28 and -1,28. The criticalvalue 1,28 corresponds to a one-sided test for positive growth at the 10 % significance level. For otherlevels of significance, choose the appr~priate critical values from a table of fractiles for the standardnormal distribution.

5

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IS 15038:2001IEC 61164(1995)

Step 4: calculate the summation:

. . .—

or

Sl=~ln(T’’/~)1=1

~type I]

Step 5: calculate the (unbiased) estimate of the parameter ~ from the formula:

$

[type II]

or

Step 6: calculate the estimate of the parameter 1 from the formula:

~= N/(T*)p

or

7.,,,

,.1$

[type I]

[type 111

[type I]

[type111

Step 7: calculate the estimated failure intensity 2(T) and mean time between failurestest time T >0, from the formulae:

1$(T), for any

6(T)= l/2(T)

NOTES .1 ~(T) and @T) are estimates of the “current” failure intensity and MTBFat time T >0, for T over the rangerepresented by the data. “Extrapolated” estimates for a future time T during the test programme, or at its expectedtermination time, may be obtained similarly, but used with the usual caution associated with extrapolation.Extrapolated estimates should not extend past the expected termination time.

.2 If the test programme is completed, then 6(T), for T = T* orthe system configuration on test at the end of the test programme.

7.2.2 Case 2 – Time data for groups of relevant failures

This alternative method is for the case where the data set

T = TN (as appropriate), estimates the MTBF of

consists of known time intervals, eachcontaining a known number of failures. It is important to note that the interval lengths and the numberof failures per interval need not be constant.

The test period is over the interval (O; T) and is partitioned into d intervals at times, O c t(1)< t(2) c... < t(d). The i-th interval is the time period between t(i- 1) and t(i)v i = 1, 2, .... d, ?(0) = O, t(d) = T.The partition times t(i) may assume any values between O and T.

Step 1: exclude non-relevant failures by reference to 7.1 of IEC 1014 and/or other appropriatedocumentation.

6

1’

IS 15038:2001IEC 61164 (1 S95)

Step 2: assemble info a data set the number of relevant failures Ni recorded in the i-th intervald

[r(i-l);t(i)], i = 1,...,d. Thetotal number ofrelevant failures is N=~Ni.1=1

For each interval, piN shall not be less than 5, (if necessary, adjacent intervals should be

combined before this test) where:

t(i)– t(i - 1)Pi =

t(d)

Step 3: for the d intervals (after combination if necessary) and

the statistic

corresponding failures Ni, calculate

d (Ni -piN)2X2=X

i=] pi N

Under the hypothesis of zero growth, that is, the failure times follow a homogeneous Poisson

process, the statistic X 2 is approximately distributed as a X2 random variable with d-1

degrees of freedom. The statistic X 2can be used to test if there is evidence of reliabilitygrowth, positive or negative, independent of the reliability growth model.

A two-sided test for positive or negative growth at the a significance level has critical value

CV=X;+; (d-1)

If X22CV

then there is evidence of positive or negative reliability growth and the analysis is continuedwith Step 4.

If X2

IS f15038 :2001IEC 61164(1995)

.-. .-

~Ni ht(i)pln t(i) – t(i-l)pln t(i– 1) 11 -in t(d) = O.1=1 t(i)~ - t(i - I)B J

Note that r(0) = O and also t(0) ~in t(0) = O All t(.) terms may be normalized with respect to t(d) and then

the final term in (t(d)) disappears. An iterative method must be used to solve this equation for ~ .

Step 5: calculate the estimate of the parameter A from the formula

i= N/t(d)B

Step 6 calculate the estimated failure intensity ;( 2’) andtest time T >0, from the formulae:

i(T) =ip T~l

6(T)=l/z(T)

NOTES

/

mean time between failures G(T), for any

.1 Z(~) and @T) are estimates of the “current” failure intensity and MTBF at time T >0, for T over the rangerepresented by the data. “Extrapolated” estimates for a future time T during the test phase, or at its expectedtermination time, may be obtained similarly, but used with the usual caution associated with extrapolation.Extrapolated estimates should not extend past the expected termination time.

.2 If the test program is completed, then @T) for T = t(d), estimates the MTBF of the system configuration ontest at the end of the test phase.

7.3 Goodness-of-fit tests

If individual failure times are available, use case 1, otherwise, use case 2.

7.3.1 Case 1 – Time data for every relevant failure

The estimation method included in 7.2.1 shall first be used to estimateCramer-von Mises statistic is then given by the following expression:

L+![(WJc2(A’f)=— 1-where

M=N and T = T* for type I tests

M= N-l and T = TN for type II tests

~

. d’.

IS 15038:2001.-—

IEC 61164(1995)

When the failure times are known, the graphical procedure described below may be used to obtainadditional information about the correspondence between the model and the data.

q

~s#,“ :,’

For the graphical procedure, an estimate of the expected time to the j-th failure,,,

E[~], is plotted

against the observed time to the j-th failure, Tj. From annex B, E[Tj] may be estimated by:

[’)lf~

‘[Tj]= ~ , Witi j=l,..., N

11~

()

JThe expected failure times, ~ , are then plotted against the observed failure times, Tj, on identical

Llinear scales, as in the example of figure A. 1. The visual agreement of these points with a line at 45°through the origin is a subjective measure of the applicability of the model.

7.3.2 Case 2 – Time data for groups of relevant failures

This test is suitable only when @ has been estimated using grouped data,, as in 7.2.2. The expected

number of failures in the time interval [t(i - 1); t(i)] is approximated by:

ei = ~[t(i)P - t(i - I)b]

For each interval, ei shall not be less than 5, and if necessary, adjacent intervals should be combined before the

test. For d intervals (after combination if necessary) and with Ni the same as in 7.2.2, calculate the statistic:

x’=i(Ni-ei)2 ...-i=l ei

The critical values of this statistic for d-2 degrees of freedom can be found in tables of the X2 k

distribution, for example in IEC 605-4 and IEC 605-6. If the critical walue at a 10 % level ofsignificance is exceeded, then the hypothesis that the power law model adequately fits the grouped datashall be rejected.

When the data set consists of known time intervals, each containing a known number of failures, thegraphical procedure described below may be used to obtain additional information about thecorrespondence between the model and the data.

For each interval endpoint t(i), the number of observed failures from O to r(i) is

N(t(i)) = ~ Nj .j=l

The expected number of failures E[N(t(i))] is estimated by

i[iV(t(i))] = L(i)b

9

IS 15038:2001IEC 61164(1995)

_——

. ..—..4

This gives&[N(t(i))l

= ~(i)k~t(i)

The graphical procedure consists of plotting

and also plotting the line

as in the example of figure A.2.

(~-l) and -ct.

For ~ O

See annex B for the relationship between in ~ and 6 and

visual agreement of these points with this line is a subjective

measure of the applicability of the model.

7.4 Confidence intervals on the shape parameter

The shape parameter ~ in the power law reliability growth model determines if the model reflectsgrowth and to what degree. If 0< ~ 1, there is negative reliability growth.

For a two-sided confidence interval on ~ when individual failure times are available, use case 1. Forgrouped failure times, use case 2.

7.4.1 Case 1 – Time data for every relevant failure

Step 1: calculate ~ from step 5 in 7.2.1

Step 2: type I test

For a two-sided 90 % confidence interval on ~, calculate

DL =X:,05;(2N)

2(N-1)

Du =X:,95; (2N)

2(N-1)

The fractiles can be found in tables of the X2 distribution, for example in IEC 605-4 and

IEC 605-6.

The lower confidence limit on ~ is

~LB=DLji

The upper confidence limit on ~ is

~uB=Duj

10

-.

IS 15038:2001 .. —

IEC 61164(1995)

One-sided 95 % lower and upper limits on ~ are pm and j3n, respectively. -–—L. -q

Type II test

For a two-sided 90 % confidence interval on ~, calculate

N“x;,~5; (2( N-1))DL =

2(N-l)(N-2)

N“X:,95; (2( N-1))DU =

2( N-l)(N-2)

The lower confidence limit on ~ is

fJLB=DL$

The upper confidence limit on ~ is

~UFj=DU”~

One-sided 95 % lower and upper limits on ~ are pm and ~n, respectively.

7.4.2 Case 2 – Time data for groups of relevant failures

These confidence interval procedures are’ suitable when ~ has been estimated from grouped data as in7.2.2.

Step 1: calculate ~ as in 7.2.2, step 4.

Step 2: calculate

P(i)t(i)

=—, with i=l, 2,...,dt(d)

Step 3: calculate the expression

[P(i)t. lnP(i)P - P(i-l)p. lnP(i-l)F~

A=~ISI P(i)p - P(i-l)@

Step 4: calculate

c=+Step 5: for an approximate two-sided 90 % confidence interval on ~, calculate

.-

~=(l,64)C

fiwhere

N is the total number of failures.

11

/

IS 15038:2001IEC 61164(1995)

Step 6: the lower confidence limit on ~ is

Pm=@(l-s)

The upper confidence limit on ~ is

One-sided 95 % lower and upper limits on ~ are pm and ~m, respectively.

7.5 Confidence intervals on current MTBF

From 7.2.1, step 7, &T) estimates the current MTBF, (3(T). For confidence intervals on 6(T) whenindividual failure times are available, use case 1. For grouped failure times, use case 2.

7.5.1 Case 1- Time data for every relevant failure

Step 1: calculate $T) from 7.2.1, step 7.

Step 2: for a two-sided 90 % confidence interval, refer to table 2, type I, or table 3, type II, andlocate the values L and U for the appropriate sample size N.

Step 3: the lower confidence limit on 6(T) is

en =L.6(T)

the upper confidence limit on @T) is

euB=u. aT)

One-sided 95 % lower and upper limits on @T) are eLB and (lm, respectively.

7.5.2 Case 2- Time data for groups of relevant failures

These confidence interval procedures are suitable when ~ has been estimated from grouped data as in7.2.2.

Step 1: calculate ~ as in 7.2.2, and calculate 6(T) as in 7.2.1, step 7

Step 2: calculate

T(i)P(i) =—, avec i=l,2,..., d

T(d)

12

d-

— -—

Step 3: calculate the expression

(d P(i)b. In P(i) P- P(i-l)P. h~(i-l)~A=~ Yj=l P(i)$ - F’(i-l)p

Step 4: calculate

K 15038:2001IEC 61164 (1995)

r1D= —+1AStep 5: for an approximate two-sided 90 % confidence interval on @T), calculate

S=(1,64). D

fiwhere

N is the total number of failures.

Step 6: the lower confidence limit on (3(T) is

eLB=ii(T) (l-s)The upper confidence limit on @T) is

e“B=6(T)(1+S)

One-sided 95 % lower and upper limits on @T) are eLB and eUB, respectively. .-

7.6 Projection technique~

The following technique is appropriate when the cotr~ctive modifications have been incorporated intothe system at the end of the test as delayed modificati~ns. The objective is to estimate the systemreliability resulting from these corrective modifications.

Step 1: separate the category A and category B failures (see IEC 1014, definitions 3.10 and 3.11).

Step 2: identify the time of first occurrence of each distinct type of failure in category B, as aseparate data set. Let 1 be the number of these distinct types.

Step 3: perform steps 1 to 5 of 7.2.1 upon this data set, in order to estimate ~, using N = I and T* orTN as applicable to the complete set of data.

Step 4: assign to each of the I distinct types of category B failures in the data set of step 2 animprovement effectiveness factor, Ei, i = 1,...1. For each of the I distinct types of category

B failures, Ei, O S Ei S 1, is an engineering assessment of the expected decrease in failure

intensity resulting from an identified corrective modification (see definition 3.1).

From these assigned values, calculate the average ~, or if preferred, postulate an averageimprovement effectiveness factor (e. g., 0,7) instead of individually assigning theEi, i=l ,...1, as described above.

13

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IS 15038:2001IEC 61164(1995)

Step 5: estimate the projected failure intensity and MTBF:

where

KA is the number of category A failures;

Ki is the number of observed failures for the i-th type of category B failures;

T = T* or TN, as used in step 3 above.

If the individual Ei values are not assigned and only the mean ~ is available, then the middle

term in the square brackets becomes:

KB(l-~)

where KB is the number of category B failures.

In this case the projected failure intensity is:

--- —-

1

“’”+.

The projected MTBF is

ep =lIZP

.,

14

. !+

_.—

IS 15038:2001IEC 61164(1995)

Table 1- Critical values for Cram6r-von Mises goodness-of-fit testat 1070 level of significance

M Critical value of statistic

3 0,154

4 0,155

5 0,160

6 0,162

7 0,165

8 0,165

9 0,167

10 0,167

11 0,169

12 0,169

13 0,169

14 0,169

15 0,169

16 0,171

17 0,171

18 0,171

19 0.171

20 0,172

30 0,172

z 60 0,173

NOTE - For type 1tests, M = N, for type 11tests, M = N-1

15

/! I!

-

.. . —

IS 15038:2001IEC 61164(1995)

Table 2 – Two-sided 90 YOconfidence intervals for MTBF from type I testing

N L u N L u

3 0.175 6,490 21 0,570 1,738

4 0,234 4,460 22 0,578 1,714

5 0,281 3,613 23 0,586 1,692

6 0,320 3,136 24 0,593 1,672

7 0,353 2,826 25 0,603 1,653

8 0,381 2,608 26 0,606 1,635

9 0,406 2,444 27 0,612 1,619

10 0,428 2,317 28 0,618 1,604

11 0,447 2,214 29 0,623 1,590

12 0,464 2,130 30 0,629 1,576

13 0,480 2,060 35 0,652 1,520

14 0,494 1,999 40 0,672 1,477

15 0,508 1,947 45 0,689 1,443

16 0,521 1,902 50 0,703 1,414

17 0,531 1,861 60 0,726 1,369

18 0,543 1,825 70 0,745 1,336

I 19 I 0,552 I 1,793 I 80 I 0,759 I 1,31120 0,56 I

I1,765 I 100 I 0,783 I 1,273

I NOTE -For N> 100L=++.05+,,+~“=+,...,5+,,2/3

I whereI U. ~ +Y12 is the 100 (0,5 + y / 2) -th fmctile of the standard normal distribution.

..— -

/

IS 15038:IEC 61164

2001

(1995)

-.—

Table 3- Two-sided 9070 confidence intervals for MTBF from type II testing

N L v21 0,6018 1,701

22 0,6091 1,680

23 0,6160 1,659

IN ILIU

13 I 0.1712 I 4,746

4 0,2587 3,825

5 0,3174 3,254

6 0,3614 2,892

7 0,3962 2,644

24 I 0,6225 I 1,790

25 0.6286 I 1,623

18 I 0.4251 t 2.463 26 0,6344 I 1,608

19 I 0.4495 I 2,324 27 0,6400 I 1,592

10 0,4706 2,216

11 0,4891 2,127

12 0,5055 2,053

13 0,5203 1,991

28 0,6452 I 1,578

29 0,6503 I 1,566

30 I 0,6551 I 1,553

35 t 0.6763 I 1.501

I 14 I 0,5337 I 1,937 40 0,6937 1,46145 0,7085 1,428

50 0,7212 1,401

60 0,7422 1,360

15 0,5459 1,891

16 0,5571 1,876

17 0,5674 1,814

18 0,5769 1,781 70 I 0,7587 I 1,327

80 ! 0.7723 I 1.303I 19 I 0,5857 I 1,752100 I 0,7938 I 1,267

,=++uo,5+,,2~.1

“=+.o,$+,,$jwhere

U0,5 +y/2 is the 100 (0,5+ y 12)-th fractileof the standardnormaldistribution.

17

i

IS 15038:2001IEC 61164(1995)

Annex A(informative)

Numerical examples

.– - -

A.1 Introduction

The following numerical examples show the use of the procedures discussed in clause 7. Table A. 1 is acomplete data set used to illustrate the reliability growth methods when the relevant failure times areknown, and table A.2 shows these data combined within intervals suitable for the grouped dataanalysis. Tables A.3 and A.4 provide data for the projection technique when corrective modificationsare delayed to the end of test. Goodness-of-fit tests, as described in 7.3, are applied when applicable.These examples may be used to validate computer programs designed to implement the methods givenin clause 7.

A.2 Current reliability assessments

The data set in table A. 1 corresponds to a test finishing at 1000 h. These data are used in the examplesof A.2. 1 and A.2.2 for type I and type II tests, respectively, and combined in table A.2 for the exampleof A.2.3 for grouped failures.

A.2. 1 Example 1: Type I test - Case 1- Time data for every relevant failure

This case is covered in 7.2.1. Data from table A.1 are used with test finishing at 1000 h.

a) Test for growth

u= -3,713. At the 0,20 significance level, the critical values for a two-sided test are 1,28 and -1,28. Since U

.

IS 15038:2001_-—

IEC 61164(1995)

A.2.2 Example 2: Type II test - Case 1 - Time data for every relevant failure;q

This case is covered in 7.2.1. Data from table A. 1 are used with test finishing at 975 h.

a) Test for growth

U = -3,764. At the 0,20 significance level, the critical values for a two-sided test are 1,28 and -1,28.Since U C– 1,28, there is evidence of positive reliability growth and the analysis is continued.

b) Parameter estimation

The estimated parameters of the power law model are:

~= 1,1067

fi = 0,5594

c) Current MTBF

The estimated current MTBF at 975 h is 33,5 h.

d) Goodness-of-fit

C2(M) = 0,041 with M = 51. At the 0,10 significance level, the critical value from table 1 is 0,173.

Since C2(M) 6,0, there is evidence of positive or negative reliability growth and the analysis is continued.

b) Parameter estimation

The estimated parameters of the power law model are:

~ =0,9615

~ = 0,5777

c) Current MTBF

The estimated current MTBF at 1000 h is 33,3 h.

d) Goodness-of-fit

X2 = 2,175 with three degrees of freedom. At the 0,10 significance level, the critical value is 6,25.

Since X2< 6,25, the power law model is accepted (see 7.3 and figure A.2).

e) Confidence interval on ~

A two-sided 90 % confidence interval on ~ is (0,3202; 0,8351).

-

19

/

1S 15038:2001IEC 61164(1995)

f) ,Conj7dence interval occurrent MTBF

A two-sided 90 % confidence interval on the current MTBF at 1000 h is ( 16,6 h; 49,9 h).

A.3 Projected reliability estimates

This example illustrates the calculation of a projected reliability estimate (see 7.6) when the correctivemodifications have been incorporated into the system at the end of test.

A.3. 1 Example 4

The basic data used in this example are given in table A.3. There are a total of N = 45 relevant failureswith KA = 13 category A failures which received no corrective modification. At the end. of the 4000 h

test, 1 = 16 distinct corrective modifications were incorporated into the system to address the KB = 32

category B failures. The category for each relevant failure is given in table A.3. Each category Bfailure type is distinguished by a number. Table A.4 provides additional information used for theprojection.

Steps in the procedure

Step 1: identify category A and B failures.

Times of occurrence and the category A and B failures are identified in table A.3. The failure times forthe 16 distinct category B types are indicated in table A.4, column 2.

Step 2: identify first occurrence of distinct category B types.

The times of first occurrence of the 16 distinct category B types are given in table A.4, column 3.

Step 3: analyze first occurrence data.

--

The data set of table A.4, column 3, is analyzed in accordance with steps 4-8 of 7.2.1. The resultsfollow below:

Parameter estimation

The estimated parameters of the power law model are:

~= 0,0326

~= 0,7472

First occurrence failure intensity estimation

The estimated current failure intensity for first occurrence of distinct category B types at 4000 h is

0;0030 h-l.

Goodness-of-fit

C*(M) = 0,085 with M = 16. At the 0,10 significance level, the critical value from table 1 is O,171.

Since C*(M) eO,171, the power law model is accepted for the times of first occurrence of distinctcategory B types.

20

/

----

IS 15038:2001IEC 61164(1995)

Step 4: assign effectiveness factors

An example of assigned individual

{

Nt~effectiveness factors for each corrective modification is given in itable A.4, column 5. The average of these 16 effectiveness factors is 0,72. An average in the range of ~

i0,65 to 0,75 is typical, based on historical experience.

Step 5: estimate projected failure intensity.

To calculate the projected failure intensity, the following values are needed:

T = 4000h

KA = 13

1 = 16

b = 0,7472

E = 0,72

Ki - table A.4, column 4

Ei – table A4, column 5

The estimated projected failure intensity at T = 4000 h (the end of test) is 0,0074 h-l.

Step 6: estimate projected MTBF.

The projected MTBF is 135,1 h.

NOTE- With no reliability growthduring the 4000 h test, tlie MTBFover this period is estimated by (4 000/45) =~-..

88,9 h. The projected MTBF is the estimated increase in MTBF due to the 16 corrective modifications and thecorresponding effectiveness factors. The sensitivity of the projected MTBF to the assigned effectiveness factors is

koften of interest. If only an average effectiveness factor of 0,60 were assigned, the projected MTBF would equal121,3 h. An average effectiveness factor of 0,80 would give a projected MTBF of 138,1 h.

21

—..—

Is 15038:2001IEC 61164(1995)

Table A.1 - Complete data - all relevant failures and accumulated test times for type I test;

F=lOOOh, N=52

2 4 10 15 18 19 20 25 39

41 43 45 47 66 88 97 104 105

I20 196 217 219 257 260 281 283 289

307 329 357 372 374 393 403 466 521

556 571 621 628 642 684 732 735 754

792 803 805 832 836 873 975

Table A.2 - Grouped data for example 3, derived from table A.1

I I I Accumulated relevant test time atGroup numb Number of failures end of group interval II 1 I 20 I I

2 13 400

3 5 600

4 8 800

5 6 1000

n9’,, .>

.

ii

/

22

)

. 4’

IS 15038:2001IEC 61164(1995)

Table A.3 - Complete data for projected estimates in example 4-

all relevant failures and accumulated test times;P=4000h, N=45, KA=13, KB=32, Z=16

Accumrdated relevant test times, Ti

Classification per category A/B, including disdnct catego~ B types

Ti 150 253 475 540 564 636 722 871 996

Category BI B2 B3 B4 B5 A B5 A B6

Ti 1003 1025 1120 1209 1255 1334 1647 1774 1927

category B7 A B8 B2 B9 B1O B9 B1O Bll

Ti 2130 2214 2293 2448 2490 2508 2601 2635 2731

Category A A A A B12 A B1 B8 A

Ti 2747 2850 3040 3154 3171 3206 3245 3249 3420

Catego~ B6 B13 B9 B4 A A B12 B1O B5

Ti 3502 3646 3649 3663 3730 3794 3890 3949 3952

Category B3 B1O A B2 B8 B14 B15 A B16

Table A.4 – Dktinct types of category B failures, from table A.3, with failure times,time of first occurrence, number observed and effectiveness factors

I Column no I1 I 2 I 3 I 4 5 I

I Type I Failure times I Theat fmt I Number I Assignedh occurrence observed effectiveness IL-1-; 2601 I 150 I 2 I 0,7 I

2I

253; 1 209; 3663I

253I

3I

0,7I

13 I 475; 3502 I 475 121 0.8 I

4 540; 3154 540 2 0,8

5 564: 722; 3420 564 3 0,9

6 996; 2747 996 2 0,9

7 1003 1003 1 0.5

18 I 1120:2635:3730 t 1120 i 3 t 0.8 i

9 1 255; 1 647; 3040 1255 3 0,9

10 i 334; 1 774; 3 249; 3646 1334 4 0,7

11 1927 1927 1 0,7

12 2 490; 3245 2490 2 0.6

13 2850 2850 1 0,6

14 3794 3794 1 0,7

15 3890 3890 1 0.7

———

-.--r

---

I,,

I 16 3952 I 3952 I 1 I 0,5

23

)

I

IS 15038:2001

IEC 61164(1995)

Expected test timeat failure

lOOOh

800 h

600h

400 h

200 h

Oh

Oh 200 h 400 h 600 h 800 h 1000h

Observed testtime at failure

Figure A.1 – Scattergram of expected and observed test times at failurebased on data of table A.1 with power law model

. ...—

.

----

24

Failures

Test time

0,20

0,10

0,09

0,08

0,07

0,06

0,05

0,04

IS 15038:2001——

,,;IEC 61164(1995)

‘-4

\4,

200 h 300 h 400h 500h 700h 900h 2000h

Accumulatedtest time

Figure A.2 - Observed and estimated accumulated failureslaccumulated test timebased on data of table A.2 with power law model

25

‘/ I I,

IS 15038:2001IEC 61164(1995)

Annex B(informative)

The power law reliability growth model - Background information

B.1 The Duane postulate

The most commonly accepted pattern for reliability growth was reported in a paper by J.T. Duane in1964. In this paper, Duane discussed his observations on failure data for a number of systems duringdevelopment testing. He observed that the accumulated number of failures N(T), divided by theaccumulated test time, T, was decreasing and fell close to a straight line when plotted against Ton in-inscale. That is, approximately,

ln(N(T)/T)=5-aln T, withd>O, a>O

Duane interpreted these plots and concluded that the accumulated number of failures is approximatedby the power law function,

fV(T)=ATp,withA>O, (3=1-aBased on this observation, Duane expressed the current instantaneous failure intensity at time T as

~N(Z’)=A~T&l, with T>O

which gives the instantaneous MTBF

The exponent cx=1- ~ is sometimes called the “growth rate”.

The Duane postulate is deterministic in the sense that it gives the expected pattern for reliability growthbut does not address the associated variability of the data.

B.2 The power law model

L.H. Crow, in 1974, considered the power law reliability growth pattern and formulated the underlyingprobabilistic model for failures as a non-homogeneous Poisson process (NHPP), {N(T), T > O}, with

mean value function

E[N(T)] = LTP

.. —-.. . -

7t’,,,,. .

*

,.-.

and intensity function

26

-/

IS 15038:2001IEC 61164(1995)

The Crow NHPP power law model has exactly the same reliability growth pattern as the Duane

postulate, for example, they both have the same expression ATP for the expected number of failures bytime T. However, the NHPP model gives the Poisson probability that N(T) will assume a particularvalue, that is, .

() n kT”AT$ ePr[N(T)=n] = ~1 ,withn =0,1,2,..,

Also, under this model

~[~Tjp]=~,with~= 1,2,...

where Tj is the accumulated time to the j-th failure.

This gives the useful first order approximation

(“1ID

+]& ; ,avec j=l,2,...

for the expected time to the j-th failure.

When ~ = 1, then z(T)s k, and the times between successive failures follow an exponential distributionwith mean l/L (homogeneous Poisson process), indicating no reliability growth. The intensity functionz(T) is decreasing for ~ c 1 (positive growth), and increasing for ~ >1 (negative growth).

The NHPP power law reliability growth model is a probabilistic interpretation of the Duane postulateand therefore allows for the development and u’se of rigorous statistical procedures for reliabilitygrowth assessments. These methods include maximum likelihood estimation of the model parametersand system reliability, confidence interval procedures and objective goodness-of-fit tests. The NHPPpower law model was extended by Crow in 1983 for reliability growth projections.

B.3 Reference documents

1 Crow, L. H., 1974, “Reliability Analysis for Complex Repairable Systems”. Reliability andBiometry, ed. F. Proschan and R.J. Serfling, pp. 379-410. Philadelphia, PA: SIAM.

2 Crow, L. H., 1983, “Reliability Growth Projection From Delayed Fixes”. Proceedings of the 1983Annual Reliability and Maintainability Symposium, pp. 84-89, Orlando, FL.

3 Duane, J.T., 1964, “Learning Curve Approach to ReliabilityAerospace 2: pp. 563-566.

Monitoring”. IEEE Transactions on

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