Accuracy and precision are two key elements of the measurement system. Often we misinterpreted both the terms considering both are the same. But in actual both are different terms altogether. Let’s discuss in this article more about differences between Accuracy vs precision.
What Is Accuracy?
Accuracy means how close your measurements are compared to the actual value. So accuracy is the deviation of the measured value from the actual value.
For example, let’s consider, you are measuring a cylinder having a diameter of 10 +/- 2 inches. If you measure the diameter as 10.2 inches, you can consider it to be accurate. But what if you measure the diameter as 12.5 inches. In this case, your measurement is not accurate.
Practically no machine can measure a perfect actual value which we call a basic dimension in drawing. Always there will be some machine error or tolerance or even due to human error it can deviate from the actual value.
Accuracy is how close you are to the actual value or target
What Is Precision?
Precision is how close your multiple measurements are for the same dimension. For example, you have multiple samples and you are measuring the same dimension. So how close all your measurements are for all samples is called precision.
Please keep in mind that, precise measurement does mean accurate measurement. You may be precise but you may not be accurate. To explain, let’s take the example again for the cylinder which has a diameter spec of 10+/-2.
Consider that, you have three sample of the cylinder, and your measurements are shown below.
- Sample 1: 12.10
- Sample 2: 12.30
- Sample 3: 12.35
So, you can see that your measurement are consistent. Your measurements are precise but those are way out of the expected actual value. That’s the reason I would say that precise measurements are not always an accurate measurement.
Repeatability and reproducibility are two critical aspects of precision. Precision tells us if your measurement system or manufacturing system is volatile or not.
Precision is how close your repeated measurements are for the same dimension
Example of Accuracy Vs Precision
Let’s consider that the outside temperature is 37-degree centigrade. You have a thermometer and that measure the temperature as 36.9 degrees. So your thermometer is giving accurate measurement.
Now consider that for the next five days the outside temperature remains 37 degrees and your thermometer keeps reading as 36.9 degrees. Then we can say that the thermometer is giving precise measurement.
Now let’s consider an example of tire pressure. Let’s consider the ideal air pressure in all four tires is 40 PSI. Now after filling the air in all your car tires, if your pressure measurements are 40, 39.9, 39.8 40, then we can say that measurement is accurate and precise. But if your measurements are like 40, 36, 38, 35, then we can say that your measurements are not precise and not accurate.
Accuracy Vs Precision: Which is more important?
Accuracy and precision both complement each other. But there is a difference between accuracy and precision. If your measurement is accurate but it is not precise then there are no benefits. On the flip side, if your measurement is precise but not accurate then also there are no benefits. So our measurement system should be accurate as well as precise to get the best out of it.
Difference between Accuracy And Precision
Let us understand the difference between accuracy and precision with few examples.
Example: 1
A sport person hitting a bulls eye
There can be four different scenario when a sportsperson hits a bulls eye
- Accurate and Precise
- Accurate but not precise
- Precise but not accurate
- Not precise and not accurate
Scenario -1: Accurate and Precise
As you can see all the shots are within the black circle which is the target. So its precise and accurate
Scenario -2 : Accurate but NOT Precise
As you can see that few shots are outside the black circle. So the shots are almost accurate but not precise
Scenario -3 : Precise but not accurate
The image shows that the shots are precise but that way out of the target, so those are not accurate
Scenario -3 : Not Precise and Not Accurate
The above image shows that none of the shots is properly on the target neither the repeated shots are close to each other. So those are neither accurate nor precise.
Example: 2
Let us take the example of the cylinder having the specified diameter 10 +/- 2 inch. Below are the measurements taken by an engineer.
Scenario-1: Accurate and precise
| Sample # | Measured Value |
|---|---|
| 1 | 10.10 |
| 2 | 10.05 |
| 3 | 10.10 |
| 4 | 10.03 |
| 5 | 10.03 |
Scenario-2: Accurate but not precise
| Sample # | Measured Value |
|---|---|
| 1 | 10.20 |
| 2 | 10.50 |
| 3 | 11.00 |
| 4 | 11.50 |
| 5 | 12.00 |
Scenario-3: Precise but not accurate
| Sample # | Measured Value |
|---|---|
| 1 | 12.10 |
| 2 | 12.15 |
| 3 | 12.05 |
| 4 | 12.20 |
| 5 | 12.03 |
Scenario-4: Not Precise and not accurate
| Sample # | Measured Value |
|---|---|
| 1 | 12.30 |
| 2 | 13.00 |
| 3 | 13.50 |
| 4 | 14.00 |
| 5 | 12.20 |
Importance of Measuring instrument in Accuracy vs precision
A good measuring instrument plays a big role in getting accurate and precise measurements. For example, if you measure a dimension with a vernier caliper and again with a micrometer, it is expected that the measurement will be more precise in micrometer due to the low least count. That is why the least count of measuring instruments is important, as this decides how precise measuring instruments are.
Must Read: What Is least count- How to calculate least count of vernier caliper
Conclusion: Precision Vs Accuracy
As I already said that both terms sound similar but from a quality point of view both are way different. We should not choose between Precision vs accuracy. We should rather try to complement both. As quality engineers, we should always focus on getting the accurate and precise measurements.
That is the only criteria for getting maximum parts accepted. Although the measuring instruments play a big role in accuracy and precision but the human error also can’t be ignored.
That’s all I have in this article. If you have any questions regarding accuracy vs precision, please do write in the comment section and I will be happy to assist you.
Frequently Asked Questions (FAQ)
What is accuracy?
Accuracy is how close your measurements are compare to the actual value
What is precision?
Precision is how close repeated measurements are for the same dimension
Accuracy vs precision: Which is important
Both are important. We need to target for best accuracy with the best precision
