Being aware of the uncertainty associated with the measurement is a very significant concept. You should not make any measurements unless you are aware of the related uncertainty.

It is important not to confuse the terms ‘error’ and ‘uncertainty’.

Error is the difference between the measured value and the ‘true value’ of the thing being measured.

For example: Assume a stick rod of True value of 25 mm and a measured value of 24 mm, here error is 1 mm.

Uncertainty is a quantification of the doubt about the measurement result.

For example: Assume a stick rod of True value of 25 mm and a measured value of 24 mm, but the measured value depends upon environment factor that makes the doubt of plus or minus 1 mm.

So, here measured value could be 23 mm or 25 mm and uncertainty is 1 mm.

Whenever possible we try to correct for any known errors: for example, by applying corrections from calibration certificates.

But any error whose value we do not know is a source of uncertainty.

Being aware of the uncertainty associated with the measurement is a very significant concept. You should not make any measurements unless you are aware of the related uncertainty.

## Uncertainty sources

- Measuring Instrument: Measuring instruments may suffer from error.
- Environmental conditions: some measurements are affected by varying in ambient conditions. For example, Calibration of slip gauge depends upon the coefficient of thermal expansion.
- The Instrument or gauge’ being measured may change over time.
- The Measurement procedure may be difficult to perform.
- Operator’s skill: Some calibration procedure requires experience and/or judgment skill.

## How to Calculate 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.

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 A 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.

The main steps to evaluating the overall uncertainty of measurement are as follows.

1.Decide what you need to find out from your measurements. Decide what actual measurements and calculations are needed to produce the final result.

2. Carry out the measurements needed.

3. Estimate the uncertainty of each input quantity that feeds into the final result. Express all uncertainties in similar terms.

4. 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.

5. Calculate the result of your measurement (including any known corrections for things such as calibration).

6. Find the combined standard uncertainty from all the individual aspects.

7. Express the uncertainty in terms of a coverage factor together with a size of the uncertainty interval and state a level of confidence.

8. Write down the measurement result and the uncertainty and state how you got both of these.

Uncertainty contributions must be expressed in similar terms before they are combined. Thus, all the uncertainties must be given in the same units, and at the same level of confidence.

## How to reduce Uncertainty?

It is important to reduce uncertainty for an accurate measurement. One simple method of reducing uncertainty is to repeat the measurement and find the average measured value.

Here some points can help you with more confidence in the measurement process,

- Reduce the random effect by repeating the measurement process
- Use the best Measuring instruments
- Always use the calibrated instrument for measurement
- Apply correction if you know any systematic effect
- Record the most common uncertainty components
- Avoid mistakes by checking calculations
- Check your measurement by different operator or method.

In general National standards have the lowest uncertainty followed by calibration laboratories and Industrial instruments.

Learn more about Uncertainty Measurement in our Online Course.