Efficacy means the same as effectiveness in most cases – the ability to produce an intended or desired result. The term is commonly used when talking about a medical treatment, a medication, a vaccine, or a plan. Before a company decides on implementing proposed changes, it should evaluate their efficacy in producing results.
In fiction stories and witchcraft, the word refers to how effective a spell or some kind of magic might be, as in “Before you go away, I would like to use the magic. I need to check the spell’s efficacy.”
The term is more commonly used in the world of pharmacology and human & animal medicine, than in business situations.
The word ‘efficacy’ has existed in the English language for 500 years. (Data Source: etymonline.com)
According to Oxford Dictionaries, efficacy means: “The ability to produce a desired or intended result.”
– Luminous Efficacy: a measure of how well a light source produces visible light. It is the ratio of luminous flux to power. Not all wavelengths are equally visible to the human eye. In this sense, the term when referring to a source defines how well it converts energy to electromagnetic radiation, and how well this emitted radiation is visible to humans.
– Self-Efficacy: our beliefs about our capabilities to produce designated levels of performance that have an impact on events that affect our lives. These beliefs determine how well you think, feel, motivate yourself, and behave.
It is the extent or strength in your belief in your own ability to reach goals or complete tasks.
If your self-efficacy is high, you are more likely to stick to a diet or workout regimen. It determines whether people chose to take on a challenging task or never try because they think it is impossible.
Effectiveness vs. efficacy
In the world of medicine and pharmacology, efficacy has to do with the observed effect in a randomized clinical trial, while effectiveness refers more to a large range of outcomes or environmental outcomes. Hence, effectiveness has a wider scope.
Stats.stackexchange.com offers these two definitions which show the difference between the two terms:
“Efficacy is the extent to which a drug has the ability to bring about its intended effect under ideal circumstances, such as in a randomized clinical trial.”
“Effectiveness is the extent to which a drug achieves its intended effect in the usual clinical setting.”