! PROBIT model for deriving a score to predict
a 0/1 outcome, e.g. default on a loan.
Dependent variable should be 0/1.
PROBIT assumes score() has a standard Normal
distribution, thus probability that
dependent variable i is in fact 1
= @PSN( score(i));
! Keywords: PROBIT, discriminant analysis, scoring,
credit scoring, maximum likelihood estimation;
SETS:
obs: score;
var: w;
oxv(obs,var): x;
ENDSETS DATA:
! Reference: Greene, W.(1997), Econometric Analysis,
Prentice-Hall, NJ, ISBN 0-02-346602-2.
Soln: w = 1.626, 0.052, 1.426, -7.452;
cbnd = 14;!Assume coefs satisfy: -cbnd < w < cbnd;
ndep = 4; ! Which column is the dependent variable.
w(ndep) = constant term of regression;
obs = 1..32;
var = gpa tuce psi grade;
x = 2.66 20 0 0
2.89 22 0 0
3.28 24 0 0
2.92 12 0 0
4.00 21 0 1
2.86 17 0 0
2.76 17 0 0
2.87 21 0 0
3.03 25 0 0
3.92 29 0 1
2.63 20 0 0
3.32 23 0 0
3.57 23 0 0
3.26 25 0 1
3.53 26 0 0
2.74 19 0 0
2.75 25 0 0
2.83 19 0 0
3.12 23 1 0
3.16 25 1 1
2.06 22 1 0
3.62 28 1 1
2.89 14 1 0
3.51 26 1 0
3.54 24 1 1
2.83 27 1 1
3.39 17 1 1
2.67 24 1 0
3.65 21 1 1
4.00 23 1 1
3.10 21 1 0
2.39 19 1 1;
ENDDATA
! Put bounds on coefficients.
These may be restrictive;
@FOR(var(k): @BND(-cbnd,w(k),cbnd););
@FOR(obs(i):
@FREE( score(i));
score(i) = w(ndep)
+ @SUM(var(k)| k#ne# ndep: w(k)*x(i,k));
);
! Maximize the log likelihood function;
MAX = @SUM( obs(i)| x(i,ndep) #le# 0:
@LOG(1-@PSN(score(i))))
+ @SUM( obs(i)| x(i,ndep) #gt# 0:
@LOG(@PSN(score(i))));
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