AN A320 CAPT; BEFORE TKOF AT IAD; WAS DISTR BY A PAX PROB AND DID NOT CATCH THE IOE FO'S ERROR ON THEIR PERFORMANCE CALCULATION.

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DURING DEP FROM IAD; A PERFORMANCE ERROR RESULTED FROM ENTERING THE WRONG INTXN INTO THE ACARS. THE THRESHOLD WAS DISPLACED 3500 FT DUE TO CONSTRUCTION. BECAUSE OF THIS DISPLACEMENT; TKOF OCCURRED WITH 1800 FT REMAINING. THE FO; AN IOE; ENTERED THE RWY INTO THE PERFORMANCE PAGE OF ACARS AS RWY 19L. THIS RESULTED IN A CALCULATED TKOF OF REDUCED PWR. HAD THE CORRECT RWY INTXN K1 OR RWY 19L BEEN ENTERED; TKOF WOULD HAVE BEEN SHOWN TO BE INADEQUATE FOR OUR WT. ALTHOUGH I WOULD HAVE BEEN IN THE LOOP AND DOUBLECHKED HIS WORK; I WAS PREOCCUPIED WITH A PAX SIT IN THE CABIN AND FAILED TO DO SO. I DID NOTICE; HOWEVER; THAT IT CALLED FOR A HIGHER SETTING THAN FOR RWY 19R GIVING ME AN INCORRECT ASSUMPTION THE DATA WAS CORRECT. THE TKOF WAS UNEVENTFUL; BUT BECAUSE OF THE LENGTHY TKOF ROLL; I RE-EXAMINED THE DATA AFTER CLBOUT AND NOTICED THE ERROR. A DISCUSSION OF PARTIAL RWY PERFORMANCE ENTRY INTO THE ACARS WAS MADE WITH THE FO. PERHAPS A REMOVAL OF THE DATA AVAILABLE FROM ACARS FOR TOTAL LENGTH AVAILABLE WHEN LENGTHY CONSTRUCTION TAKES PLACE WOULD ELIMINATE THIS TYPE OF ERROR. PERMISSION WAS OBTAINED FROM MY FO TO SUBMIT THIS RPT ON OUR BEHALF.