Probable cause to develop coefficient A
If there is a constant deviation but magnetic compass is well sitted. Then this is due to coefficient apparent A.
Constant LHA error.
Faulty Azimuth mirror.
Faulty pivot point for compass card.
Lubber line is not in line with the F&A line of the ship.
NS line of compass card is not in line with NS axis of compass needle.
Establish compass error by comparing magnetic compass heading and gyro compass heading.
If vessel swing too quickly. Insufficient steading time results in hysteresis effect.