Which statement correctly describes efficacy and effectiveness in behavior-change research?

• A. Efficacy tests whether an intervention works under ideal conditions; effectiveness tests whether it works in real-world settings
• B. Efficacy is about cost; effectiveness is about time
• C. They are the same thing
• D. Efficacy is real-world; effectiveness is lab

Answer: (A)

In behavior-change research, you distinguish whether an intervention works in principle versus in real-world practice. Efficacy asks if the intervention can work under ideal, highly controlled conditions—participants are carefully selected, adherence is encouraged and monitored, and everything is tuned to maximize the chance of seeing an effect. Effectiveness asks if the intervention works in real-world settings with typical clients, varying environments, and ordinary adherence and resource constraints. This makes the statement that efficacy tests ideal conditions and effectiveness tests real-world settings the best description. The other ideas miss the distinction: one focuses on cost or time, which aren’t the constructs here; they claim the two concepts are the same; or they reverse which one is real-world and which is lab-based.