A LOW TIME PLT HAD AN NMAC WITH A STUDENT PLT AT IWS.

Narrative

PLTING ACFT X DOING PATTERN WORK. ON CLBOUT AFTER TOUCH-AND-GO; I HEARD ANOTHER ACFT (Y) CALLING 1.5 MI FINAL. I TURNED XWIND; THEN DOWNWIND; ANNOUNCING MY LOCATION AT ALL TIMES. ON DOWNWIND; OPPOSITE LNDG END OF RWY 15; BEFORE STARTING DSCNT; I SAW AN ACFT LOW AND ON APPROX 1/4 MI FINAL. SO I ASSUMED THIS WAS THE ACFT WHO PREVIOUSLY HAD ADVISED A 1.5 MI FINAL. I DSNDED AND ANNOUNCED BASE. AT APPROX 500 FT; STARTED TO TURN FINAL WHEN I SAW ANOTHER ACFT; ALREADY ON FINAL AND DSNDING TOWARDS ME. I IMMEDIATELY STOPPED THE TURN TO FINAL AND DSNDED RAPIDLY FOR APPROX 75 FT AND EXITED THE PATTERN TO THE UPWIND SIDE. ACFT Z RESPONDED BY A GAR; AFTER I HAD CALLED ON RADIO WARNING OF DANGER. PLT OF ACFT #2 WAS A STUDENT PLT WHO ADVISED HAD ANNOUNCED THE 1.5 MI FINAL. PROB AROSE DUE TO INSUFFICIENT COM FROM ACFT #2. I MADE 3 POS CALLS; WHILE ACFT Z WAS SILENT ON LONG FINAL. IF ACFT Z HAD ADVISED STILL ON FINAL; I COULD HAVE EXTENDED DOWNWIND LEG. ACFT Z WAS PROBABLY ON A 2.5 MI FINAL; POSSIBLY DUE TO INEXPERIENCE OF THE STUDENT PLT; DISTANCE WAS INACCURATELY ESTIMATED; AND POS NOT UPDATED. CORRECTIVE ACTION: TO MAKE IT A PRACTICE TO UPDATE POS ON FINAL; ESPECIALLY WHEN EXTENDED FINAL AND OTHER TFC IN THE PATTERN.