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SUBROUTINE PROBT

PURPOSE
   TO OBTAIN MAXIMUM LIKELIHOOD ESTIMATES FOR THE PARAMETERS A
   AND B IN THE PROBIT EQUATION  Y = A + BX.  AN ITERATIVE
   SCHEME IS USED.  THE INPUT TO THE SUBROUTINE CONSISTS OF K
   DIFFERENT DOSAGE LEVELS APPLIED TO K GROUPS OF SUBJECTS, AND
   THE NUMBER OF SUBJECTS IN EACH GROUP RESPONDING TO THE
   RESPECTIVE DOSAGE OF THE DRUG.

USAGE
   CALL PROBT (K,X,S,R,LOG,ANS,W1,W2,IER)

DESCRIPTION OF PARAMETERS
   K   - NUMBER OF DIFFERENT DOSE LEVELS OF THE DRUG.  K SHOULD
	 BE GREATER THAN 2.
   X   - INPUT VECTOR OF LENGTH K CONTAINING THE DOSE LEVEL OF
	 THE DRUG TESTED.  X MUST BE NON-NEGATIVE.
   S   - INPUT VECTOR OF LENGTH K CONTAINING THE NUMBER OF
	 SUBJECTS TESTED AT EACH DOSE LEVEL
   R   - INPUT VECTOR OF LENGTH K CONTAINING THE NUMBER OF
	 SUBJECTS AT EACH LEVEL RESPONDING TO THE DRUG
   LOG - INPUT OPTION CODE
	 1- IF IT IS DESIRED TO CONVERT THE DOSE LEVELS TO
	    COMMON LOGARITHMS.	THE DOSAGE LEVELS SHOULD BE
	    NON-NULL IN THIS CASE.
	 0- IF NO CONVERSION IS DESIRED
   ANS - OUTPUT VECTOR OF LENGTH 4 CONTAINING THE FOLLOWING
	 RESULTS
	 ANS(1)- ESTIMATE OF THE INTERCEPT CONSTANT A
	 ANS(2)- ESTIMATE OF THE PROBIT REGRESSION COEFFICIENT
		 B
	 ANS(3)- CHI-SQUARED VALUE FOR A TEST OF SIGNIFICANCE
		 OF THE FINAL PROBIT EQUATION
	 ANS(4)- DEGREES OF FREEDOM FOR THE CHI-SQUARE
		 STATISTIC
   W1  - OUTPUT VECTOR OF LENGTH K CONTAINING THE PROPORTIONS
	 OF SUBJECTS RESPONDING TO THE VARIOUS DOSE LEVELS OF
	 THE DRUG
   W2  - OUTPUT VECTOR OF LENGTH K CONTAINING THE VALUES OF THE
	 EXPECTED PROBIT FOR THE VARIOUS LEVELS OF A DRUG
   IER - 1 IF K IS NOT GREATER THAN 2.
	 2 IF SOME DOSAGE LEVEL IS NEGATIVE, OR IF THE INPUT
	   OPTION CODE LOG IS 1 AND SOME DOSAGE LEVEL IS ZERO.
	 3 IF SOME ELEMENT OF S IS NOT POSITIVE.
	 4 IF NUMBER OF SUBJECTS RESPONDING IS GREATER THAN
	 NUMBER OF SUBJECTS TESTED.
	 ONLY IF IER IS ZERO IS A PROBIT ANALYSIS PERFORMED.
	 OTHERWISE, ANS, W1, AND W2 ARE SET TO ZERO.

REMARKS
   THE PROGRAM WILL ITERATE ON THE PROBIT EQUATION UNTIL TWO
   SUCCESSIVE SOLUTIONS PRODUCE CHANGES OF LESS THAN 10**(-7).

SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
   NDTR
   NDTRI

METHOD
   REFER TO D. J. FINNEY, PROBIT ANALYSIS. 2ND ED. (CAMBRIDGE,
   1952)