This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. The property of being effective

n. The ability to be effective.

n. The degree to which something is effective.

n. The quality of being effective.

n. The quality of being effective.

n. power to be effective; the quality of being able to bring about an effect

n. capacity to produce strong physiological or chemical effects

"But the key to their effectiveness is their rattle, and it's not just the noise, but also how it is made."

power    powerfulness   

efficacious    effectual    effectivity    effect    efficacity    effector    efficacy    effectuate    efficiency    efficient   

causativeness    effectuality    efficaciousness    efficiency    point    forcefulness    cogency    virtue    duty   

ineffectiveness   

efficiency    reliability    efficacy    flexibility    accuracy    productivity    capability    integrity    completeness    complexity