When I hear the word “efficiency” I’m thinking of engines and light. How much energy does the car use for actually bringing me from A to B and how much is wasted on heat or simply transporting the car itself? For light bulbs, how much energy is used for producing light, and how much for unwanted heat?
Efficiency is the ratio of resources spent for a task and total resources spent in the process, including non-task related stuff. The resources can be either energy, time, or other.
Effectiveness on the other hand is about goal-alignment. You want to make sure that you satisfy the actual client needs. The client might ask you to implement a neural network for figuring out a good layout for the website because they have heard that neural networks can solve any task. Being effective might be to ask if they might want a responsive design.
Peter F. Drucker summarized the difference between effectiveness and efficiency pretty well:
Efficiency is doing the thing right. Effectiveness is doing the right thing.
As an example, let’s say you go into a coffee shop and you order a cappuccino:
| Ineffective | Effective | |
|---|---|---|
| Inefficient | You receive a coke after 20 minutes | You receive your cappuccino after 20 minutes |
| Efficient | You receive a coke within 10 seconds | You receive your cappuccino within 10 seconds |
Efficient vs Effective: All 4 combinations are possible.