noun
Refers to the quality of being valid, effective, or producing the intended result. Often used in formal or technical contexts such as law, medicine, and policy.
この治療法の有効性はまだ証明されていない。
The effectiveness of this treatment has not yet been proven.
契約
I consulted a lawyer about the validity of the contract.
効果 means 'effect' or 'result', focusing on the outcome itself, whereas 有効性 emphasizes the property of being effective.
Compound of 有効 (valid, effective) + 性 (-ness, -ity).