Efficacy vs. Efficiency: Difference Between Results-Based Words

Updated November 1, 2021
Team members Efficacy vs Efficiency vs Effectiveness
    Team members Efficacy vs Efficiency vs Effectiveness
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    Used under Getty Images license

It's important to use both efficacy and efficiency in your daily life. Although these words sound very similar, they describe two different attributes of a task. Learn more about efficacy vs. efficiency, as well as how they relate to the concept of effectiveness.

Using Efficacy and Efficiency

It seems as though efficacy and efficiency have the same meaning. After all, they both relate to doing a job well. But their key differences are in their definitions.

  • efficacy - being able to achieve results
  • efficiency - being able to achieve results without wasting resources

For example, when you're able to make a sandwich, you're using efficacy because you've achieved the desired result (a delicious sandwich). When you make a sandwich in a short amount of time without spilling mustard everywhere, you're using efficiency. You need both in order to have your sandwich on time!


How to Use Efficacy

The word efficacy often appears in scientific and skill-based contexts. It's the first and most basic skill required in getting a job done. Efficacy can appear as efficacious when used as an adjective.

  • Soccer players need to master efficacy in passing the ball to each other.
  • Wearing a thin t-shirt is not an efficacious way to stay dry in the rain.
  • The medicine proved its efficacy by easing my headache.

What Is Self-Efficacy?

Self-efficacy refers to a person's belief that they can achieve results. It's a specific type of self-confidence that lets someone know they have the ability to complete a task. For example:

  • Toddlers develop self-efficacy by mastering crawling before trying to walk.
  • Teachers help their students with self-efficacy in writing by allowing them to edit their own work.
  • People with high self-efficacy aren't afraid of a challenge.

How to Use Efficiency

You're probably more familiar with efficiency, which describes the ability to complete a task quickly and without using too many resources. It's a step above efficacy, which is concerned with being able to complete the task at all. Examples of efficiency (or the adjective efficient) in a sentence include:

  • Our company values efficiency, so don't spend too much time on one project.
  • Using a timer can help children to get dressed with efficiency.
  • Making your lunch at night is an efficient way to prepare for the next day.

Efficacy vs. Efficiency vs. Effectiveness

Efficacy and efficiency share the Latin root efficere, which means "to work out or complete." But there's another word from this root that often gets confused: effectiveness. When you're studying the effectiveness of an action, you're seeing how well it worked based on results.

efficacyIf a shampoo cleans your hair, it's efficacious.
efficiencyIf a shampoo cleans your hair in only one minute, it's efficient.
effectivenessIf a shampoo cleans your hair really well, it's effective.

Medical Usage

When you hear the terms "vaccine efficacy" and "vaccine effectiveness," they're referring to two different concepts. The medical world (especially the field of epidemiology) uses efficacy and effectiveness to indicate different clinical conditions. Their definitions in an epidemiological context are:

  • efficacy - how well a medicine or vaccine works in a controlled condition
  • effectiveness - how well a medicine or vaccine works in a real-world condition

The word efficiency has the same meaning in the medical world as it does in everyday speech. "Vaccine efficiency" would refer to how quickly a vaccine can reduce the spread of a disease.

Be Efficacious and Efficient in Your Vocabulary

Choosing the right word to use immediately adds both efficacy and efficiency to your writing. After all, it shows that you can communicate in the fewest words possible! Keep the vocabulary lesson going with a guide to implicit vs. explicit.