# SAMPLE SELECTION BIAS IN CREDIT SCORING MODELS

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1 SAMPLE SELECTION BIAS IN CREDIT SCORING MODELS John Banasik, Jonathan Crook Credit Research Centre, University of Edinburgh Lyn Thomas University of Southampton ssm0

2 The Problem We wish to estimate an accept-reject model applicable to ALL applicants but we only know the performance of those accepted in the past. Conventional Treatment Reject Inference Extrapolation Multinomial regression (various) Augmentation Others ssm1

3 Weaknesses of Reject Inference Extrapolation (Hand & Henley 1993) Provided population model for all applicants is same as that for accepts only then Direct posterior modelling (Logistic, Probit) does not give biased estimates of parameters. Linear Discriminant Analysis will give biased estimates because LDA assumes p(x G) and p(x B) distributions are normal but they are not. ssm2

4 A More General Approach to Missing Data Let D=(D 0, D m ) D denotes whether a case has defaulted subscripts: 0 denotes value is observed m denotes missing Let S = {0,1} 0 denotes value of D is missing 1 denotes value of D is observed Little & Rubin s (1987) Categories Missing Completely At Random (MCAR): S is independent of D and X Missing at Random(MAR): S depends on X but not on D Non-ignorably Missing (NIM): S depends on D and on X ssm3

5 MCAR D 0 values are a random sample from the D values. Estimated parameters from this sample does not give biased estimates of parameters applicable to population of all D values MAR Can be shown (Hand & Henley, Little& Rubin) that if sampling is conditional only on X then using only observed values of D will not give biased estimates of the parameters of a population model applicable to all D values NIM D depends on X values and on other variables (called them Z). So S depends on X and Z. Relating D 0 values only to X WILL give biased estimates of population of all applicants model because the chance of observation of D values depends on Z and X not just X. Z values appear in the likelihood function and so ignoring them by relating D 0 merely to X will lead to omitted variable bias. Known as sample selection bias in the econometric literature. ssm5

6 Sample Selection Techniques We are interested in parameterising a model relating to a population where Y 1 = β 1 X 1 + ε 1....(1) We observe only those values of Y 1 when another variable, Y 2 is positive Suppose Y 2 is a linear model Y 2 = β 2 X 2 + ε 2 Call observed values of Y 1, Y* 1. Then: Y 1 = Y* 1 if Y 2 > 0 Y 1 is unobserved if Y 2 0 If we apply OLS to eqtn 1 using only the selected sample the regression line is E(Y* 1 X 1, Y 2 > 0) = β 1 X 1 + E(ε 1 ε 2 > - β 2 X 2 ) which differs from the regression for the population of interest E(Y 1 ) = β 1 X 1 ssm6

7 Further effect Suppose there are variables in X 2 (and so affect Y 2 ) but they are not in X 1 (they do not affect Y 1 ) Is possible that if estimate Eqtn 1 using only the selected sample these variables may appear statistically significant when in fact they do not affect Y 1. ssm7

8 Bivariate Probit Model with Sample Selection ssm8 Suppose Y 3 and Y 4 are continuous unobserved variables with Y 3 = β 3 X 3 + ε 3 Y 4 = β 4 X 4 + ε 4 and Y 3 =1 if Y 3 > 0 0 if Y 3 0 and Y 4 =1 if Y 4 > 0 0 if Y 4 0 with E(ε 3 ) = E(ε 4 ) Cov(ε 3,ε 4 ) = ρ (ε 3,ε 4 ) ~ bivariate normal Also Y* 3 is observed only if Y* 4 = 1 Y* 4 is observed in all cases Appears to be the credit scoring problem

9 Information Observed Good Bad Total Accepted observed A observed B observed C Rejected not observed D not observed E observed F Given that (ε 3, ε 4 ) ~ bivariate probit the unconstrained observations with associated probabilities are Y* 3 =1 Y* 4 =1: P(Y* 3 =1 Y* 4 =1) = Φ B (β 3 X 3, β 4 X 4,ρ) Cell A Y* 3 =0, Y* 4 =1: P(Y* 3 =0, Y* 4 =1) = Φ B (-β 3 X 3, β 4 X 4, -ρ) Cell B Y* 4 =0 : P(Y* 4 =0) =Φ U ( β 4 X 4 ) Cell F where Φ B is CDF of bivarariate normal distribution with density φ B (β 3 X 3, β 4 X 4,ρ) Φ U is CDF of univariate normal distribution with density φ U ( β 4 X 4 ) ssm9

10 Notice One cause of ρ=0 is if at least one variable is omitted from both the default and the AR equation or if the omitted variables are correlated Sample selection bias may occur unless ρ=0 If accept-reject decision is deterministic (no overrides) and if the accept-reject model can be estimated perfectly then ρ=0 and no sample selection bias occurs But overrides do occur the accept-reject model may not be perfectly estimatable (eg may have been estimated by neural network algorithm) ssm10

11 Previous Studies Boyes, et al (1989) Used bivariate probit to estimate parameters in a default equation but do not compare the predictive performance of this model with one based on aacepts only using a holdout of all applicants Greene (1992, 1998) Compares the predicted conditional probabilities ( P(Y* 3 X 3, Y 4 =1) with unconditional probabilities - but does not compare the predictive performance using a sample of all applicants Other Relevant Studies

12 Methodology: Initial Analysis Data 1. Complete sample comprises applicants virtually all of whom are accepted, distinguishing those who would normally be accepted. 2. Course classification of 46 variables (typically 6 categories each): Classification designed to be equally applicable to All applicants Accepted applicants in isolation Each variable to be considered alternatively as Weights of evidence Sets of binary variables 3. Training sample obtained by proportional stratified sampling to enhance comparability with holdout sample. 2/3 of all applicants in the training sample 2/3 of all accepted applicants in the training sample 2/3 of all good applicants in the training sample 2/3 of all good accepted applicants in the training sample

13 Methodology: Initial Analysis Model Estimation and Evaluation 1. All explanatory variables deployed to estimate logistic regression model using both types of variable (weights of evidence and binary sets) on the basis of training cases alone for both All applicants Accepted applicants only Weights of evidence separately estimated for both applicant categories on the basis of training cases. 2. Each model used to predict corresponding holdout cases to assess the possible extent of over-fitting. 3. The model for accepted applicants only used to predict all holdout cases to assess its general applicability. Preliminary indications are that Models based on accepted applicants only seem generally applicable Reject inference affords very modest prospects for improved predictive performance.

15 Table 4: Comparative Prediction Performance of the Bivariate Probit Model Ref Bivariate Probit With Selection Probit (Accepts only) Probit (All cases) Variables Number Number Number Included of Cases Rho Signif AUROC of Cases AUROC of Cases AUROC Binary Variables LL35 All LL30 Those in SW LL31 SW less Weights of Evidence LL36 All LL33 Those in SW LL34 SW less

16 Methodology: Subsequent Analysis General Approach Subsequent analysis empirically examines two aspects of the foregoing conclusions. Perhaps the limited scope for reject inference occurs only because the acceptance threshold is too low. The variable sets for models previously used to accept or reject applicants may diverge from those of subsequent models used to predict how good or bad will be the accepted applicants. To examine these aspects 1. The data set was recast to distinguish 5 acceptance thresholds, one for each couple of deciles. The same proportional stratified sampling was adopted. The original course classification was retained. Weights of evidence were re-estimated for each training sample. 2. Distinct Accept-Reject and Good-Bad models were constructed on the basis of distinct explanatory variable sets.

17 Methodology: Subsequent Analysis Distinct Variable Sets The Accept-Reject model was based on 2540 Scottish applicants. The remaining 9668 English applicants comprised 6446 training cases to estimate a Good-Bad model and 3222 holdout cases. 1. An eligible pool of variables for the Accept-Reject model was discerned by stepwise regressions where Accept-Reject was the dependent variable, one each for Scotland, England, and the UK. 2. A variable surviving in any of these three was entered into a stepwise regression on Scottish data where Good-Bad was the dependent variable. 3. The Scottish model based only on significant variables was used to provide a ranking of acceptability used to group English applicants into 5 bands. 4. An English model was discerned on the basis of stepwise regression where Good-Bad was the dependent variable. This determined the variable set to be used for each of the five bands.

18 Table 5: Variables Included in the Accept-Reject and Good-Bad Equations Good-Bad Accept-Reject Ref Variable Equation Equation 43 B1 20 Time and Present Add 33 B2 40 Weeks since last CCJ 22 No of children under B3 11 Television area code 32 B4 31 P1 35 P2 6 B5 19 Accommodation Type 17 Age of applicant (years) 26 Has Telephone 23 P3 34 B6 21 Type of Bank/Building Soc Accts 16 B7 28 Current Electoral Role Category 38 P4 25 Occupation Code 36 B8 30 Years on Electoral Role at current address 48 No of searches in last 6 months 47 B9 7 B10 9 B11 46 B12 44 B13 27 P5 Bn = bureau variable n; P = proprietary variable n; denotes variable is included

21 Table 8: Comparative Prediction Performance of the Bivariate Probit Model for each Band Ref Bivariate Probit With Selection Probit (Accepts only) Probit (All cases) Weights of Variables Number Number Number Evidence Included of Cases Rho Signif AUROC of Cases AUROC of Cases AUROC LL39 Band 1 All Band Band Band Band LL42 Band 1 As Table Band Band Band Band LL43 Band 1 As Table 5 less Band Band Band Band

22 Conclusions The scope for improved predictive performance by any form of reject inference is modest The scope depends on the cut-off score adopted. Use of the bivariate probit model with selection only marginally improves predictive performance although this depends on the variables included in the model.

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