C #DPOSA V1A 29-JUN-73. C LAST UPDATE: C FUNCTION DPOSA (NEVENT, TIME, AVSEP) C C *PURPOSE. C TO CALCULATE THE PROBABILITY OF "NEVENT" EVENTS OCCURING C IN TIME "TIME" WHEN THE PROBABILITY DISTRIBUTION FOR THE C NUMBER OF EVENTS IS OF THE POISSON TYPE WITH A MEAN TIME C BETWEEN EVENTS OF "AVSEP". C NOTE THAT THIS IS A PROBABILITY DENSITY FUNCTION NOT A C CUMULATIVE PROBABILITY. THAT IS THE PROBABILITY IS FOR C EXACTLY "NEVENT" EVENTS OCCURRING. C C *PARAMETERS: C C NEVENT - INTEGER VALUE OF THE NUMBER OF EVENTS FOR WHICH THE C PROBABILITY IS REQUIRED. C TIME - REAL VALUE OF TIME INTERVAL FOR WHICH PROBABILITY C IS REQUIRED. C AVSEP - REAL VALUE OF MEAN TIME BETWEEN EVENTS FOR THE C POISSON DISTRIBUTION. C C INPUT PARAMETERS: C NEVENT, TIME, AVSEP C OUTPUT PARAMETERS: C (FUNCTION VALUE ONLY) C C *METHOD. C EVALUATES THE POISSON DISTRIBUTION PROBABILITY DENSITY C FUNCTION FOR "NEVENT" EVENTS OCCURRING WHEN THE MEAN IS C "TIME/AVSEP" . C C *ACCURACY. C LIMITED ONLY BY ACCURACY OF THE MACHINE. C C *RESTRICTIONS. C C *ERROR CONDITIONS. C IF "NEVENT" IS LESS THAN ZERO OR "TIME/AVSEP" IS LESS THAN C OR EQUAL TO ZERO THE VALUE WILL BE RETURNED AS ZERO. C C *NON STANDARD ROUTINES CALLED. C FCTOR - FACTORIAL OF INTEGER ARGUMENT - MRMLIB. C C *TYPICAL TIMES. C C *ORIGIN. M.R.MANNING. C C *COMMENTS. C THIS PROBABILITY APPLIES TO A WIDE CLASS OF SITUATIONS. C BASICALLY WHENEVER THE EVENTS ARE UNRELATED, SUCH AS FAILURE C OF COMPONENTS, RADIOACTIVE DECAY ETC. ETC. SEE P.R.BEVINGTON C "DATA REDUCTION AND ANALYSIS..", OR ANY GOOD TEXT ON STATISTICS. C C #END. C DPOSA= 0. IF ( NEVENT .LT. 0) RETURN A= TIME/AVSEP IF ( A .LE. 0.) RETURN DPOSA= A**NEVENT*EXP(-A)/FCTOR(NEVENT) C RETURN END