Measurement uncertainty is the expression of the statistical dispersion of the values attributed to a measured quantity. All measurements are subject to uncertainty and a measurement result is complete only when it is accompanied by a statement of the associated uncertainty, such as the standard deviation. By international agreement, this uncertainty has a probabilistic basis and reflects incomplete knowledge of the quantity value. It is a non-negative parameter.
The measurement uncertainty is often taken as the standard deviation of a state-of-knowledge probability distribution over the possible values that could be attributed to a measured quantity. Relative uncertainty is the measurement uncertainty relative to the magnitude of a particular single choice for the value for the measured quantity, when this choice is nonzero. This particular single choice is usually called the measured value, which may be optimal in some well-defined sense (e.g., a mean, median, or mode). Thus, the relative measurement uncertainty is the measurement uncertainty divided by the absolute value of the measured value, when the measured value is not zero.
Measurement uncertainty is a quantitative indication of the quality of measurement results, without which they could not be compared between themselves, with specified reference values or to a standard. Uncertainty evaluation is essential to guarantee the metrological traceability of measurement results and to ensure that they are accurate and reliable. In addition, measurement uncertainty must be considered whenever a decision has to be taken based on measurement results, such as in accept/reject or pass/fail processes.
Estimating measurement uncertainty can be a difficult task. Especially, since most measurement uncertainty guides do not give you a process or procedure.
Specify the Measurement Process and Equation
Before you dive in and begin calculating uncertainty, it is best to have a plan. The first part of your plan should be to identify the measurement process or system that you wish to evaluate.
Identify and Characterize the Uncertainty Sources
Now that you have determined the measurement process that you are going to evaluate, you need to identify the factors that influence uncertainty in measurement results.
This process is not typically easy and can be very frustrating. So, stay calm, be patient, and keep researching. You may be surprised by how many influences can affect your measurement results.
Quantify the Magnitude of Uncertainty Components
Before calculating measurement uncertainty, you must first determine the magnitude of each contributing factor. To accomplish this, you may need to perform some data reduction and analysis.
How to Quantify Uncertainty
To quantify uncertainty, you need to follow the four steps below:
- Collect Information and Data
- Evaluate and Select the Right Data
- Analyze the Data
- Quantify Uncertainty Components
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