Designing multiple biometric systems: Measure of ensemble effectiveness

Designing multiple biometric systems:

Measure of ensemble effectiveness

Allen TangOPLab @ NTUIM

Agenda

Introduction Measures of performance Measures of ensemble effectiveness Combination Rules Experimental Results Conclusion

2

INTRODUCTION

Introduction

Multimodal biometrics is better

Fuse multiple biometric results

Fusion at matching level is easier

4

Introduction

Which biometric experts shall we choose?

How to evaluate ensemble effectiveness?

Which measure gives out the best result?

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MEASURES OF PERFORMANCE

Measures of performance

Notation E={E1…Ej…EN}: a set of N experts U={ui}: the set of users sj: the set of all scores by Ej for all user sij: the score by Ej for a user ui

fj(ui): function of Ej produce sij for ui

th: threshold; gen: genuine; imp: impostor

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Measures of performance: Basic

False Rejection Rate(FRR) for expert Ej:

False Acceptance Rate(FAR) for expert Ej:

......(1))|()|()( genthsPdsgenspthFRR jth

jjj

......(2))|()|()( impthsPdsimpspthFAR jth

jjj

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Measures of performance: Basic

p(sj|gen): Ej score probability distribution to genuine users

p(sj|imp): Ej score probability distribution to impostor users

Threshold(th) changes with the requirements of the application at hand

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Area under the ROC curve(AUC)

Equal error rate(ERR)

The “decidability” index d’

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Measures of performance: AUC

Estimate AUC by Mann-Whitney statistics:

This formulation of AUC is also called the “probability of correct pair-wise ranking”, as it computes the probability P( > )

......(3))(

]),([1 1

,,

nn

ssIAUC

n

p

n

q

impjq

genjp

genjps ,

impjqs ,

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n+/n−: no. of genuine/imposter users : score set by Ej for genuine users : score set by Ej for impostor users

, ,

, , , ,

, ,

1:

( , ) 0.5 :

0 :

gen impp j q j

gen imp gen impp j q j p j q j

gen impp j q j

s s

I s s s s

s s

genjps ,

impjqs ,

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Features of AUC estimated by WMW stat. :

Theoretically equivalent to the value by integrating ROC curve

Attain more reliable estimation of AUC in real cases(finite samples)

Divide all scores sij into 2 sets: &

genjps ,

impjqs ,

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Measures of performance: EER

EER is the point of ROC curve where FAR and FRR are equal

The lower the value of EER, the better the performance of a biometric system

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Measures of performance: d’

The d’ in the biometrics is to measure the separability of the distributions of genuine and impostor scores

2 2

'2 2

gen imp

gen imp

d

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Measures of performance: d’

μgen/μimp: mean of genuine/impostor score distribution

σgen/σimp: std. deviation of genuine/impostor score distribution

The larger the d’, the better the performance of a biometric system

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MEASURES OF ENSEMBLE EFFECTIVENESS

Measures of ensemble effectiveness

4 measures for estimating effectiveness of ensemble of biometric experts: AUC, EER, d’, and Score Dissimilarity(SD) Index

But we must take the difference in performance among the experts into consideration

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Generic, weighted and normalized performance measure(pm) formulation:

pmδ=μpm∙ (1−tanh(σpm))

For AUC: AUCδ=μAUC∙ (1−tanh(σAUC)) The higher the AUC average, the

better the performances of an ensemble of experts

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For ERR: ERRδ=μERR∙ (1−tanh(σERR)) The lower the ERR average, the better the

performances of an ensemble of experts For d’, consider the value of d’ that can be

much larger than 1, use normalized D’=logb(1+d’) instead of d’, and base b=10 according to the values of d’ in experiments

Thus D’δ=μD’∙ (1−tanh(σD’)) is used

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Measures of ensemble effectiveness: SD index

SD index is based on the WMW formulation of the AUC, and is designed to measure the amount of improvement in AUC of the combination of an ensemble of experts

SD index is a measure of the amount of AUC that can be “recovered” by exploiting the complementarity of the experts

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Consider 2 experts E1 & E2, and all possible scores pairs , divide these pairs into 4 subsets S00, S10, S01, S11:

,1 ,1 ,2 ,2{{ , },{ , }}gen imp gen impp q p qs s s s

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AUC of E1 & E2 are listed below, where card(Suv) is the cardinality of the subset Suv:

SD index is defined as:

11 101

[ ( ) ( )]card S card SAUC n n

11 012

[ ( ) ( )]card S card SAUC n n

10 01

11 10 01

( ) ( )......(4)

( ) ( ) ( )

card S card SSD

card S card S card S

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The higher the value of SD, the higher the maximum AUC that could be obtained by the combined scores

But actual increments of AUC depends on the combination method, and high SDs usually related to low performance experts

Performance measure formulation for SD: SDδ=μSD∙ (1−tanh(σSD))

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COMBINATION RULES

Combination Rules

Combination(Fusion) in this work is at the score level, as it is the most widely used and flexible combination level

Investigate the performance of 4 combination methods: mean rule, product rule, linear combination by LDA, and DSS

LDA & DSS require a training phase to estimate the parameters needed to perform the combination

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Combination Rules: Mean Rule

The mean rule is applied directly to the matching scores produced by the set of N experts

,1

1 N

i mean ijj

S sN

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Combination Rules: Product Rule

The product rule is applied directly to the matching scores produced by the set of N experts

,1

1 N

i prod ijj

S SN

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Combination Rules: Linear Combination by LDA

Linear discriminant analysis(LDA) can be used to compute the weights of a linear combination of the scores

This rule is to attain a fused score with minimum within-class variations and maximum between-class variations

,

ti LDA iS W S

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Combination Rules: Linear Combination by LDA

Wt(W): transformation vector

computed using a training set Si: vector of the scores assigned to

the user ui by all the experts μgen/μimp: mean of genuine/impostor

score distribution Sw: within-class scatter matrix

1( )W gen impW S

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Combination Rules: DSS

Dynamic score selection(DSS) is to select one of the scores sij available for each user ui, instead of fusing them into a new score

The ideal selector is based on the knowledge of the state of nature of each user:

i

,*i

max{ }: if u is a genuine......(5)

min{ }: if u is an imposter

ij

iij

sS

s

32


DSS selects the scores according estimation of the state of nature for each user, and the algorithm is based on quadratic discriminant classifier (QDC)

For the estimation, a vector space is built where the vector components are the scores assigned to the user by the N experts

33


Train a classifier on this vector space by using a training set related to genuine and impostor users

Using the classifier to estimate the state of nature of the user

After getting the estimation of the state of nature of the user, select user’s score according to (5).

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EXPERIMENTAL RESULTS

Experimental Results: Goal

Investigate the correlation between the measures of the effectiveness of the ensemble

Understand final performances achieved by the combined experts, and get the best measures

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Experimental Results: Preparation

Scores source: 41 experts and 4 DBs from open category in 3rd Fingerprint Verification Competition(FVC2004)

No. of scores: For each sensor and for each expert, a total of 7750 scores, attempts from gen./imp. users are 2800/4950

For LDA & DSS training, divide scores into 4 subsets, with 700 gen. and 1238 imp. each

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Experimental Results: Process

No. of expert pairs: 13,120(41x40x2x4) For each pair, compute the measures

of effectiveness by AUC, EER, d’ and SD index

Combine the pairs using 4 combination rules, then compute related values of AUC and EER to show the performance

Use a graphical representation of the results of the experiments

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Experimental Results: AUCδ plotted against AUC

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According to graphs, AUCδ isn’t useful because no clear relationship with AUC of combination rules

High AUCδ attains high AUC, but lower AUCδ gets value in wide range

High AUCδ relates to high performance and similar behavior experts pair

Mean rule has best AUCδ

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Experimental Results: AUCδ plotted against EER

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AUCδ is uncorrelated with the EER too Any value of AUCδ , the EER spans

over a wide range of values Can not predict the performance of

the combination in terms of EER by AUCδ

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Experimental Results: EERδ plotted against AUC

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Behavior better than AUCδ, but still no clear relationship between EERδ and AUC

Mean rules has best result too

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Experimental Results: EERδ plotted against EER

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No correlation between EERδ and EER Graphs from AUCδ against EER and

EERδ against EER have similar results So AUC and EER are not suitable to

evaluate combination of experts, despite that they are widely used for unimodal biometric system

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Experimental Results: D’δ plotted against AUC

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Higher values of D'δ guarantee smaller ranges of values of the performance of the combination

D'δ has higher and clearer correlation with performance of combination

Mean rule gets best result, and product rule is the worst

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Experimental Results: D’δ plotted against EER

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D'δ has better correlation with EER too

D'δ is much better than AUCδ and EERδ

D'δ is a good measure to evaluate the effectiveness of candidate ensembles of biometric experts

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Experimental Results: SDδ plotted against AUC

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SDδ does have some correlation with AUC because SD is designed to predict max improvement in AUC by combining experts, but is still not clear enough

Small SDδs guarantee large performance, especially for high performance experts pair, because higher the AUC of the individual experts, the smaller the complementarity

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Experimental Results: SDδ plotted against EER

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SDδ with EER isn’t as good as AUC

Result from product rule is still no good

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CONCLUSION

Conclusion

To predict performance improvement, product rule exhibit worst, mean rule is best, and LDA & DSS not far from mean rule

Under mean rule, LDA & DSS have similar results

Performance of combined experts is not highly correlated with single one in general

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Conclusion

The best measure of ensemble is D'δ, while AUC δ and ERR δ isn’t good enough, and SD δ performs like AUC δ

Based on above results, D' δ with mean rule tops any other pairs of measure and combination rule, and is the most suitable method to be the measure of ensemble effeectiveness

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Designing multiple biometric systems: Measure of ensemble effectiveness