Unlike the VOR it is non directional (as the name suggests) and thus can not have any error. To check your aircraft's system you select "test" and move the pointer away from the indicated direction before releasing the selector and checking that the pointer returns to the expected bearing.
...hmmmm...
A dangerous thought for those that do not understand how MW signals can be bent.
Sure, there are failure conditions in VORs which do not produce a fail flag (not least the famous time when Alan Mann Avionics wired up my OBS to give reverse indications on both VOR and ILS) but at least some failures are trapped either by the TST ident or the fail flag.
On the ADF there is simply no way of telling that the direction from which the signal being received is the straight line to the NDB.
I am not going to regurgitate ATPL radio theory, but night effects, coastal diffraction, mountain effect, dip, thunderstorms, reflections/multi-path, quadrantal error all add up to an alarmingly inaccurate and unreliable aid.
It seems a shame that, because GPS is demonstrably imperfect, the CAA are chucking it out with the bathwater. It is
much better than NDB/ADF.
The CAA polices by consent. When they take a position so radically opposed to that of many pilots who (in possession of all the facts) do prefer GPS over ADF, they risk their ability to regulate and control, in the same way that 20mph speed limits are ignored.