B737-700
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Confusion
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Human Factors
Shortly after takeoff from DAL; Fort Worth Center cleared us direct to PGS on the TYSSN 5 Arrival. We were originally filed on the TYSSN 5.IGM. I manually entered PGS at the top of LEGS Page 1. I then had to delete the old points from the IGM transition. When completed; I did not notice that the CEJAY and KADDY points did not load. Later on; I failed to do the arrival briefing; which would have captured this error. When I looked at the LEGS Page; I saw what I expected to see; not what was actually there. During the arrival; LA Center told us we were north of course (because we were going PGS direct to TYSSN instead of PGS.CEJAY.KADDY.TYSSN). LA Center cleared us direct to KADDY; and we had no problem making the 12;000 feet and 250 knots KADDY restriction. Incidentally; before we figured out the missing waypoints error; we thought our north-of-course error was due to an intermittent GPS-L error message that began near the White Sands Missile Range. We assumed GPS jamming was in effect and had caused the navigation error. But we later discovered the error on the LEGS Page.  Expectation bias led me to being complacent about a required descent briefing. Had I done the briefing completely; the error would have been easily captured.
B-737 Captain reported a course deviation caused by an error in loading the STAR into the FMS.