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NDTR.FOR
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1985-11-29
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46 lines
C
C.......................................................................
C
C SUBROUTINE NDTR
C
C PURPOSE
C COMPUTES Y = P(X) = PROBABILITY THAT THE RANDOM VARIABLE U,
C DISTRIBUTED NORMALLY(0,1), IS LESS THAN OR EQUAL TO X.
C F(X), THE ORDINATE OF THE NORMAL DENSITY AT X, IS ALSO
C COMPUTED.
C
C USAGE
C CALL NDTR(X,P,D)
C
C DESCRIPTION OF PARAMETERS
C X--INPUT SCALAR FOR WHICH P(X) IS COMPUTED.
C P--OUTPUT PROBABILITY.
C D--OUTPUT DENSITY.
C
C REMARKS
C MAXIMUM ERROR IS 0.0000007.
C
C SUBROUTINES AND SUBPROGRAMS REQUIRED
C NONE
C
C METHOD
C BASED ON APPROXIMATIONS IN C. HASTINGS, APPROXIMATIONS FOR
C DIGITAL COMPUTERS, PRINCETON UNIV. PRESS, PRINCETON, N.J.,
C 1955. SEE EQUATION 26.2.17, HANDBOOK OF MATHEMATICAL
C FUNCTIONS, ABRAMOWITZ AND STEGUN, DOVER PUBLICATIONS, INC.,
C NEW YORK.
C
C.......................................................................
C
SUBROUTINE NDTR(X,P,D)
C
AX=ABS(X)
T=1.0/(1.0+.2316419*AX)
D=0.3989423*EXP(-X*X/2.0)
P = 1.0 - D*T*((((1.330274*T - 1.821256)*T + 1.781478)*T -
1 0.3565638)*T + 0.3193815)
IF(X)1,2,2
1 P=1.0-P
2 RETURN
END