PASS/FAIL CALCULATION RESULTS IN PDF FORMS

I have created an interactive adobe pdf form using Adobe 9 pro. Calculating average and standard deviation of five number sets. Then trying to dispaly PASS/FAIL results compared to a prset value, The calculations works just fine. When entering a data point to change average deviation and standard deviation that exceeds the criteraia, the PASS/FAIL does not immidately works. I have to enter the wrong data point several times before the PASS/FAIL works. Also when deviation is less than set value (PASS), I have to renter the correct value of original data set several times before the PASS is desplayed again,

var x=this.getField ('dev");

event.value=x<=set?"PASS":"FAIL";

Appreciate any comments.