BAD : British farmers receive only a little aid since their farms are relatively effective.
GOOD : British farmers receive only a little aid since their farms are relatively efficient.
See note at EFFICIENT (↑ efficient )
BAD : Aspirin is very effective to relieve pain.
GOOD : Aspirin is very effective in relieving pain.
be effective in doing sth : 'Doctors soon realized that this drug was also effective in relieving the symptoms of arthritis.'