Chapter 6
How to calculate uncertainty of measurement
6.1 The two ways to estimate uncertainties
6.2 Eight main steps to evaluating uncertainty
to calculate the uncertainty of a measurement, firstly you must identify the sources of uncertainty in the measurement. Then you must estimate the size of the uncertainty from each source. Finally the individual uncertainties are combined to give an overall figure.
There are clear rules for assessing the contribution from each uncertainty, and for combining these together.
To calculate the uncertainty of a measurement, firstly you must identify the sources of uncertainty in the measurement. Then you must estimate the size of the uncertainty from each source. Finally the individual uncertainties are combined to give an overall figure.
There are clear rules for assessing the contribution from each uncertainty, and for combining these together.
6.1 The two ways to estimate uncertainties
No matter what are the sources of your uncertainties, there are two approaches to estimating them: "Type A" and "Type B" evaluations. In most measurement situations, uncertainty evaluations of both types are needed.
Type evaluations - uncertainty estimates using statistics (usually from repeated readings)
Type B evaluations - uncertainty estimates from any other information. This could be information from past experience of the measurements, from calibration certificates, manufacturer’s specifications, from calculations, from published information, and from common sense.
There is a temptation to think of "Type A" as "random" and "Type B" as "systematic", but this is not necessarily true.
How to use the information from Type A and Type B evaluations is described below.
6.2 Eight main steps to evaluating uncertainty
The main steps to evaluating the overall uncertainty of a measurement are as follows.
- Decide what you need to find out from your measurements. Decide what actual measurements and calculations are needed to produce the final result.
- Carry out the measurements needed.
- Estimate the uncertainty of each input quantity that feeds into the final result. Express all uncertainties in similar terms. (See section 7.1).
- Decide whether the errors of the input quantities are independent of each other. If you think not, then some extra calculations or information are needed. (See correlation in Section 7.3.)
- Calculate the result of your measurement (including any known corrections for things such as calibration).
- Find the combined standard uncertainty from all the individual aspects. (See Section 7.2)
- Express the uncertainty in terms of coverage factor (see section 7.4), together with a size of the uncertainty interval, and state a level of confidence.
Write down the measurement result and the uncertainty, and state how you got both of these. (See Section 8.)
This is a general outline of the process. An example where these steps are carried out is given in Section 9.
2007-07-22, 05:22 PM
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