Estimating the uncertainty of measurement certainly sounds like an oxymoron. Basically you need to measure the uncertainty of measurement. Still with me? ISO 17025:2017 accounts the uncertainty of measurement under the sub-clause 5.4.6 where it is prescribed for labs to understand, document, and apply procedures to estimate the uncertainty of measurement well.
The essential requirements related to the uncertainty of measurement are stated below:
- It is mandatory for labs to make use of appropriate procedures to estimate the uncertainty of measurement.
- Methods used that are statistical are not always be used to estimate the uncertainty of measurement.
- All the components of the uncertainty shall be identified and estimated to the closest match for each of the product testing.
- The appropriate estimation of uncertainty of measurement shall be based on knowledge of the performance of the methods based on previous experience.
- Validation data methodology can be used to determine the uncertainty of measurement.
The necessary part of measuring the uncertainty of measurement is to identify all sources of uncertainty of measurement and to seek every component no matter how minor it is if it’s contributing to the uncertainty of measurement and its estimation so the contributing factors must be taken into consideration.
Few Basic Concepts Relevant to Uncertainty of Measurement
Let’s brush up ourselves on some beginner’s guide terminology related to the uncertainty of measurement.
A measurement is nothing but a number given to a specific property which is being made with the help of an instrument like Rulers, Stopwatches, Weighing scales, or Thermometers.
By measuring, we can tell how heavy or how to light an object is or how long it is. There are two parts of measuring anything. One is the number (magnitude), and the other is unit (the nature of the measurement).
It is the specific quantity subject to measurement.
Uncertainty of measurement
Uncertainty of measurement is about quality of measurement. It is the doubt that always exists about the outcome of the measurement. Even the measuring instrument is made with high precision and accuracy; there will always be a doubt.
It is the difference between the measured value and the actual value of the thing being measured. Error and Uncertainty of measurement are not the same terminologies. On the other hand, uncertainty is the doubt about result measurement.
Sources of Errors and Uncertainties
- The measuring instrument
- The item being measured
- The Measurement Process
- Calibration method
- Operators’ skill and competencies
- Sampling Issues
- The Environment
General Kinds of Uncertainty in any Measurement
Random uncertainty of measurement occurs when, after repeating a measurement, you get a different random result.
Systematic Uncertainty of Measurement occurs where the same influence
It is described as the uncertainty of the outcome of a measurement which is also expressed as a Standard Deviation.
Combined Standard Uncertainty
It is described as the result of a measurement which is estimated standard deviation that is equivalent to the positive square root of total variance obtained by adding all the uncertainty components.
It is set of operational standards that are set up under most specified conditions, the relation between quantities’ values indicated by the instrument which is used to measure or any values represented by reference material and corresponding values obtained by the standard.
The difference between the expected outcome of testing and the real accepted value obtained.
Procedure for proving characteristics of the capacity of the method which is appropriate for solving any analysis problem.
A material or a substance used for the calibration of a measuring apparatus or assessment of a measurement method or for assigning values to the material.
Essential Aspects to Estimate Uncertainty of Measurement
Following are the steps of the procedure for estimating the uncertainty of measurement:
Measurand needs to be defined
Defining the measurand crystal clear is essential. The correlation between the measurand and input quantity should be established through the model equation.
Identification of Sources of the uncertainty of measurement
Enlisted below are few examples of probable sources of uncertainty of measurement:
- Unclear defining criteria of the measurement
- Sampling method
- The way sample being transported and stored
- Sample preparation method
- Internal Lab conditions and External environmental conditions under which measurement has been carried out
- Lab operators
- Any change in the testing procedures
- Equipment that is used to get measurement done
- Reference material
Stepwise Method to Estimate Uncertainty of Measurement According to ISO 17025
Following is the eight-step approach to estimate the uncertainty of measurement:
- First, decide what you need to conclude from your measurement. If possible, draw an estimate about actual measurement output and calculations that are necessary to produce the outcome.
- Having done the above step, conduct the measurements as per your need.
- Draw an estimate of each uncertainty of each input quantity that provides a feed to the final desired result.
- Then decide and evaluate if the errors present in the input quantities are independent to each other or not. If the errors present in the input quantities are dependent on each other, then you need to do some more calculations.
- Calculate the outcome of your measurement exercise.
- Evaluate the combined standard uncertainty from all the individual aspects.
- Denote uncertainty with uncertainty interval and state level of confidence.
- Mention measurement results along with uncertainty and also state how you get the measurement is done and the uncertainty of measurement.
How to Minimize Uncertainty in Measurement?
Here are a few simple stepwise techniques that can be used to minimize uncertainty in measurement:
- Calibration of Measuring Instrument: It is crucial to perform a calibration on measuring instrument before measuring anything with the help of measuring instrument.
- Do Corrections: Make corrections that can affect your measurement practice. For example: if you find zero error in your measuring instrument then make your measurement outcome likewise after removing zero error.
- Sync your Measurements to National Standards: Make your measurement outcomes traceable and in sync with the national standards.
- Measuring Instrument: Choose the best of the best measuring instrument and make the maximum use of calibration facilities provided to the lab.
- Repeat Measurement Process: Check the accuracy and precision of measurement by repeating them again and again until to get closer results as possible.
- Cross those numbers or the values of the constants that are you had to copy from some reference.
- Be aware of the fact that during multiple calibration chains, the uncertainty increases after every each step.