If you find it hard to remember when to use "efficacy" and when "efficiency" is preferred, then the explanations we prepared for you below will certainly help you. In fact, it is actually easy to remember when you are spelling it correctly and when it's a misspelling according to the context where you use this pair of words, once you clearly understand what each defines.
So let's take a closer look to the definitions of "efficacy" and "efficiency" to make sure this confusion never occurs again!
Efficacy vs. Efficiency
The truth is, the real reason why "efficacy" is so often confused for "efficiency" is because they do have slightly similar meanings. They both refer, somehow, to how efficient an intervention or an action is. Even so, there are some subtleties that make these nouns different and appropriate in different contexts.
"Efficacy" defines the performance of an intervention in an ideal context, in controlled circumstances. Or, more concretely, it refers to the ability of something to produce the desired results, in ideal conditions. "Efficiency" refers to the ability of doing something well and obtaining the best possible results with the minimum or null waste of time, money, energy or other resources.
When do we use "efficacy"?
"Efficacy" expresses the ability of a domain, of a person or of an action, to obtain the desired results, in an ideal context.
Example: The efficacy of medicine was impressive in quickly creating a vaccine for the new disease.
When do we use "efficiency"?
"Efficiency" defines the ability to obtain the best possible results with the minimum waste of resources. It can be used in any domain, from personal time/task management, to the business field and even entire industries.
Example: The manager's efficiency in administrating the financial, energy and time resources for the best performance of his team resulted in a great profit for the organization and a minimum waste of money.
"Efficacy" refers to the ability of achieving certain results in certain conditions, whereas "efficiency" defines the ability to obtain the best possible results with the minimum waste of resources. While both nouns measure performance, the slight difference in their meanings is quite important when choosing your spelling, especially in a formal conversation where choosing your words is important. Once you understand what each represents, anyway, it will be easy to never misspell or confuse "efficacy" for "efficiency" again!