Simple:
* The efficacy of the new medication was proven in clinical trials.
* The study questioned the efficacy of the traditional treatment method.
More complex:
* The researchers are investigating the efficacy of different learning strategies for students with dyslexia.
* The government is facing scrutiny over the efficacy of its economic policies.
* The efficacy of the new marketing campaign is being closely monitored by the company.
With synonyms:
* The effectiveness of the new software was immediately apparent.
* The study aimed to assess the power of the herbal remedy.
* The effectiveness of the exercise program was remarkable.
Note: "Efficacy" is often used in formal contexts, especially when discussing scientific research, medical treatments, or policy decisions.