Are you familiar with the differences and relationships between Accuracy, Error, Tolerance, and Uncertainty in calibration Results?
There are so many terms that we always use or read during our measurement process. Knowledge in these terms is the key to proper understanding and execution of your measurement results which is also an important part of a calibration awareness training that you should consider.
I have recently received the below question to most of the comments in my posts, And therefore, it is worth a topic to discuss.
Q1. I am unable to understand the relation between Accuracy, Error, and uncertainty. Can you tell me with example?
Q2. What is the difference between Tolerance and Uncertainty?
One way to easily learn, implement the results in a calibration certificate, and to properly understand most of the calibration procedure is to understand the measurement terms in it.
In my last article, I have presented the difference between Calibration, Verification, and Validation in the measurement process. See this link if you did not read it yet >> Calibration-Verification-Validation
Now in this article, I will present the difference, relationships and Interpretations of the following terms: Accuracy, Tolerance, Error, and Uncertainty.
Moreover, I will share with you below topics to answer the questions above:
- The Difference Between Accuracy and Error (Accuracy vs Error)
- The difference between Error and Uncertainty (Error vs Uncertainty)
- The Difference between Tolerance and Uncertainty (Tolerance vs Uncertainty)
- The relationships between Accuracy, Error, Tolerance, and Uncertainty in Calibration Results
As per JCGM 200 and 106: 2012, below are the actual definitions:
- Accuracy = closeness of agreement between a measured quantity value and a true quantity value of a measurand
- Error or measurement error = measured quantity value minus a reference quantity value
- Tolerance =difference between upper and lower tolerance limits
- Uncertainty or measurement uncertainty = non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used- make more meaningful
First Let me present each term in a simple way that I understand (I hope for you too)
Accuracy is the closeness of UUC results to the STD (true) value. This ‘closeness’ is usually represented in percentage value (%) and can be shown in the same unit by converting it into an error value ( %error). The more close the percentage value to ZERO (0%), the more accurate.
Accuracy is more on a qualitative description which means that it does not present an exact value.
Accuracy is equivalent to or a percent error (%error). This is where the value of error will be used.
Accuracy can be calculated using the formula:
If you know the error from a specific range, you can calculate the accuracy.
Error is simply the difference between the UUC and STD results after calibration. It has the same unit as the measured parameter.
See the below example from a calibration result based on the photo above.
It is the maximum error or deviation that is allowed or accepted in the design of the user for its manufactured product or components.
Tolerance is a range of values that is acceptable or permitted by the user from the result of the process or product measurement.
It is the permitted error that are:
- Calculated from the process design by the user
- Prescribed by regulatory bodies (based on Accuracy Class)
- Manufacturer specifications (based on Accuracy)
The formula is Upper limit – lower limit (UTL-LTL)
UTL or LTL is the value based on tolerance limits. Tolerance limits = Tolerance/2
Tolerance Limits are provided either by manufacturer or process requirements.
If we perform a measurement, the value of tolerance limit will tell us if the measurement we have is acceptable or not.
If you know the calibration tolerance limits, it will help you answer the questions like:
1. How do you know that your measurement result is within the acceptable range?
2. Is the final product specification pass or fail?
3. Do we need to perform adjustments?
“The wider the tolerance Interval, the more product or measurement results will pass or accepted.”
I will only explain here what measurement uncertainty is, not how to calculate measurement uncertainty.
This will be in a different post because there are many processes involved before we can come up with a single expanded uncertainty result. My point here is to show you the difference and relationships of uncertainty results with the other measurement terms.
Ok let’s go…
Uncertainty or Measurement Uncertainty is defined as the quantification of the doubt. There is always a doubt that exists, an error included in the final result that we do not know, therefore, there are no perfectly exact measurement results,
Why do we have uncertainty or doubt in our measurement?
Some of the main reasons why we have doubt or uncertainty in measurements are:
- Inadequate knowledge of the Effects of the environmental conditions on the measurement;
- Personal bias in reading analog instruments, an example is the resolution or smallest value that you can read.;
- inexact values of measurement standards and reference materials;
- approximations and assumptions incorporated in the measurement method and procedure;
- variations in repeated observations of the measurand under apparently identical conditions – Repeatability
You can read more under JCGM 100:2008 also known as the GUM
We do not know this error that is added to our measurement results, and therefore, we cannot remove or correct it.
Because we cannot correct it, what we can do is to determine or estimate the range where the true value is located, this range of true value is the measurement uncertainty result.
See below image with calibration result as an example:
“The smaller the measurement uncertainty, the more accurate or exact our measurement results.”
Differences Between Accuracy, Error, and Tolerance, their Relationships and Interpretations in Measurements
Accuracy Versus Error
Accuracy is for gauging how small/large the error is (a qualitative description), while the Error is the actual representation of accuracy in the same units as the measured parameter (measurand). In other words, the error shows the quantity of accuracy in the unit of measurement used.
Accuracy and error have opposite relationships (indirectly proportional) but they are directly related to each other.
Accuracy is a qualitative form, meaning no exact value or measurement result is presented, only a presentation (usually in percentage form) of how good or bad or how far and near but no exact value, while error shows the absolute value or actual value.
In order to show the exact or absolute value, we need to use the error. The error shows the exact distance of the measurement result from the true value.
Sometimes, accuracy is presented in a quantitative form which is actually the error at a certain range. See below example based on the photo above:
Example: accuracy of 0.1% of Full scale is more accurate than a 0.5% of full scale reading.
@Full scale = 5000 psi
For 0.1% accuracy = 0. 1%*5000 = 0.001*5000 = 5 psi
For 0.5% accuracy = 0.5% *5000 = 0.005*5000 = 25 psi
From the results above, it is clear that 5 psi is a much smaller error compared to 25 psi, which means that 0.1% is more accurate than 0.5%
Accuracy Class and MPE: A Manufacturer Tolerance Value
At this point, accuracy can be used as a Tolerance based on manufacturer specifications. We call this type of accuracy as the Accuracy Class or Grade.
Then from accuracy class, it is calculated to its equivalent error which is called the MPE (maximum permissible error) as required by standards like ASTM and ISO or manufacturer’s specifications. This is then used as the tolerance limits, and afterward, the tolerance value. See the below image presentation.
The Difference Between Error and Uncertainty and its Relationship in Measurement Results
Error vs Uncertainty
As we know now, Error is the difference between UUC – STD reading. The smaller the error, the more accurate the measurement results.
UUC -STD = error
Any error that we know can be corrected. From calibration certificate results, where a standard value is given, we can now determine the error. And since the error is determined, we can correct it by either adding or subtracting the correction factor which is the opposite of the error.
Correction = STD-UUC.
From the table above, we now know that the error is a +3, or more than 3, therefore, in order to achieve the most accurate result during use in measurement, we need to remove the excess 3, hence minus 3. Now, the final value of our measurement result is 497.
In reality, the exact error is not known, therefore, what we can do is to estimate it. This estimated error is the measurement uncertainty. A range of estimated errors.
Uncertainty is the ‘range of values’ where the true value or actual location of the measurement results (UUC) lie, while the Error is the ‘exact result’ of the difference between the UUC and STD which shows how accurate the measurement result is by showing the actual distance to the true (STD) value.
Uncertainty is a boundary within the measurement results to show the range of its actual location based on a given confidence level (95%, k=2)). See the below photo.
When the uncertainty results are included in the measurement results, we are 95% sure that the true value lies somewhere in the range 496.1 to 503.9 psi.
The smaller the measurement uncertainty, the more accurate the result, because it shows that the range of estimated errors are very small.
The Difference Between Tolerance and Uncertainty
Based on the image above, it shows the difference between Tolerance and Uncertainty, such as:
- The Tolerance is the permissible value of errors that are limited by the upper and lower tolerance limits (see the description above);
- The Uncertainty shows the boundary or limits of an estimated error where the exact measurement result’s location.
Calibration tolerance limits are provided by the user, by regulatory bodies or as per specifications while Uncertainty is calculated based on the combined errors related to the STD and UUC.
With a given Tolerance and Uncertainty, TUR (Test Uncertainty Ratio) can be calculated. A ratio of 4:1 is recommended.
The formula for TUR is equal to Tolerance / (2 x expanded uncertainty) or Tolerance Limit / Expanded Uncertainty. See the below formula and example. The TUR for this is equal to 6.4:1
Tolerance and Uncertainty As a Basis for Decision Rule as per ISO 17025:2017
Uncertainty is used or included when determining compliance with specifications as per the requirement of ISO 17025:2017. One way to tell if a product has passed or failed based on a given tolerance, a decision rule.
What is a Decision Rule?
As per ISO 17025:2017, clause 3.7, it is a rule that describes how measurement uncertainty is accounted for when stating conformity with a specified requirement.
One way to account for measurement uncertainty is to include it in the measurement results. We will use it as if it is a tolerance limit within the measured value. See the calculation on the below photo:
Below are the decision rules (as an example based on the image below):
- If the results of our measurements are within the tolerance indicated including the uncertainty results, then it is a ‘pass’.
- If the results of our measurements are outside the tolerance including the uncertainty results, then it is considered ‘fail’ or ‘out of tolerance’.
- If one of the uncertainty limits is outside the tolerance while the other limits are inside the tolerance limit, then it is not a pass or a fail, we call it ‘Indeterminate’. The decision now is based on the user.
The requirement is that, during the assessment of the statement of conformity, we should consider the uncertainty results and apply the above decision rule. See the below presentation to explain more:
Based on the above results/presentation, it is “passed” because the result (UUC reading) including the uncertainty results is inside the tolerance limits.
For a detailed explanation regarding the ‘decision rule’ and ‘reporting statement of conformity’, I have created a separate post. Check out this link>> 5 Steps to Implement ISO 17025 Decision Rules
The Relationships Between Accuracy, Error, Tolerance, and Uncertainty – The Interpretation from a Calibration Result
Accuracy shows the degree of closeness of a measurement result to the true or reference value.
The degree of closeness from the reference value is presented in the actual value (not a percentage (%) of) through the calculated Error (UUC-STD).
The error shows how the measurement results have deviated from the true value.
While accuracy is calculated based on error and true value, Uncertainty is calculated based on the combined errors or inaccuracy of reference standards (STD) and the measurand (UUC).
Uncertainty shows the range where the measurement results (UUC) actually located. An estimated location of true UUC value which is limited by the confidence interval (usually @ 95%, k=2).
In order for the result to be acceptable, uncertainty results should stay within the tolerance limit.
Not in all cases uncertainty is larger than the error as presented here. But in all cases to have a sure ‘Pass’ remarks, when included in measurement results, it should stay within the tolerance limit.
Tolerance shows the permissible error of measurement results and it is the difference between the UTL and LTL (UTL-LTL)
To sum it all, see below image.
I hope I have presented with you simple definitions and explanations regarding the differences, relationships, and interpretations of accuracy, error, tolerance, and uncertainty.
These are the most used terms when it comes to reporting calibration results, understanding and creating a calibration procedure or just simply understanding a calibration process.
These will help us understand more about the calibration process and therefore we can have a proper and correct way of interpretations thus leading to correct measurement implementation.
In this post, I have presented the following:
- The Difference Between Accuracy and Error ( Accuracy vs Error)
- The difference between Error and Uncertainty (Error vs Uncertainty)
- The Difference between Tolerance and Uncertainty (Tolerance vs Uncertainty)
- The relationships between Accuracy, Error, Tolerance, and Uncertainty from a Calibration Results
When it comes to decision making regarding the results of our measurements, these are the terms that we need to understand.
For the relationships between Accuracy, Precision, and Tolerance, visit my other post HERE
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