Help with validating a numerical (text) field.

On my form I have several numerical fields that must be completed.  I checked the "Required" property box, but I don't see any benefit.  So I used a "On Blur" script type with the javascript below.  This works but the function fires 3 times before the cursor is back in the field for correction.  Below is my script.

var oal = this.getField("OAL");

if (oal.value == 0)

{

app.alert('Must complete Over-All-Length field!');

oal.setFocus();

}