Accuracy vs Precision for PMP Exam (PMBOK Guide)
In Project Quality Management of the PMP® Exam, the concept of “Accuracy” and “Precision” is one of the most easily confused concepts (another equally important terms for quality management is Quality vs Grade which we will deal with in a separate article).
Accuracy vs Precision
Accuracy and Precision are descriptions of repeated measurements taken from a process. Let’s go to the definitions of them.
- Accuracy: a measure of how the measured values close to the target value
- If the measured values are accurate, they may not be precise as they can be larger or smaller than the target value on a wide range.
- Precision: a measure of how the measured values close to each other (but not necessarily close to the target value)
- If we say the measurements are very precise, we do not give out any hint on whether the measurements are accurate (i.e. close to the target value).
In order to help PMP® Aspirant understand the differences between Accuracy vs Precision, let’s consider the following case.
Suppose you are a supplier for metal wire and a client ordered five wire of 100 meters. The wire cutting machine cuts the wires according to your instruction and you measured them manually to get the following data:
- First wire: 100.6 m
- Second wire: 100.5 m
- Third wire: 100.7 m
- Fourth wire: 100.7 m
- Fifth wire: 100.5 m
How would you describe the measured values? Accurate or Precise?
Answer: The measured values are precise but NOT accurate. While they are close to each other (within ~0.2%), they are not close to the desired value of 100 meters.
What about if the following data are recorded:
- First wire: 100.2 m
- Second wire: 100.1 m
- Third wire: 99.8 m
- Fourth wire: 99.8 m
- Fifth wire: 100.0 m
Answer: The measured values are accurate but NOT quite precise. While they are close to the target (within ~0.2%), they are not as close to each other as in the first case.
Accuracy & Precision: Which is more important?
The question now is which attribute is more important in quality management? I am sure judging from the example above, you will see that accuracy is MUCH MUCH MORE important than precision. The client required wires of 100 meters in length, your job is to give wires with length as close to 100 meters as possible in order to satisfy the requirement (of course, some client would welcome if you give more than they request, that would not be considered in this case). A precision wrong length is still wrong, no matter how precise the measurements are. Only when the output is accurate then it is meaningful to strike for precision.