Effective, effectual, efficacious, efficient refer to that which is able to produce a (desired) effect.
Effective is applied to that which has the power to, or which actually does, produce an effect: an effective action, remedy, speech.
Effectual is used esp. of that which produces the effect desired or intended, or a decisive result: An effectual bombardment silenced the enemy.
Efficacious suggests the capability of achieving a certain end: an efficacious plan, medicine.
Efficient (applied also to persons) implies the skillful use of energy or industry to accomplish desired results with little waste of effort: efficient methods; an efficient manager.
effective - Definitions from Dictionary.com
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