'efficacy' is usually used when you are looking at performance in a controlled or ideal environment. 'effectiveness' is usually used when you are looking at performance in 'real-world' conditions. That is, in conditions that you would find in the every day world around you. Here's an example how the words differ in meaning. "The drug company is now doing efficacy trials for it's new vaccine" ie. The drug company is studying the performance of the vaccine in a laboratory under strict test conditions. "The drug company is now doing effectiveness trials for it's new vaccine" Here the performance is measured by simply giving it to people in the real world where conditions are not controlled or regulated.