Estimation of Uncertainty in Quality Control Tests & Results...........

Calculating Uncertainty of Measurement

Some Important Terms

1.         Uncertainty

                                 Uncertainty is deviation from the true value and can only be expressed in the form of a range, e.g. +3, +10.6, +0.07, etc.

Published Definition:

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

2.         Error Vs Uncertainty

Error expresses a problem and can only be expressed (in statistical terms) as either positive or negative value, while uncertainty cannot be expressed in the form of a single value.

Published Definition of Error:

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

3.         Sources of Uncertainty

a.         Test / Calibration Performer(s)

It is the uncertainty of the person performing the test / calibration, but as a person’s error cannot be calculated in the form of numerical values that be used in advanced combined estimations, we determine Repeatability and Reproducibility through the test/ calibration results they achieve.

Uncertainty of Repeatability:

It is the deviation found in results when an activity is repeatedly performed under the same condition and by the same person.

Uncertainty of Reproducibility:

It is the deviation found in results when an activity is repeatedly performed on the same type of sample under different conditions (in our case we suggest changing the performer only and try to sustain the rest of the uncertainty sources’ condition as much as possible).

b.         Method used

It is uncertainty expressed in the test / calibration method (equipment manual, etc.) when some assumptions have to be made due to limitation of features and/or resources.

c.         Accessories

The apparatus and accessories being used have uncertainty of their own. In most cases it can be found in their manuals, on their label or their certificates. Experienced personnel who are capable of performing test / calibration of equipment are also able to find its uncertainty, if it’s not given in any other source.

d.         Material, Sample and Chemical

Any material and/or sample being used has its own uncertainty, which is usually provided with its certificate, literature or container. If it’s not provided, personnel can calculate it by comparing it with a reference standard where uncertainty is already stated.

4.         Types of Uncertainty

a.         Type A

Value of Uncertainty achieved through statistical calculation. We use Type A for calculation of two sources of uncertainty which are Repeatability and Reproducibility.

b.         Type B

Value of Uncertainty achieved from any other source e.g. manuals, certificates, etc.

If you are interested in calculation of  Type A or B, you can further comment or just drop me a mail  on qc.macandrains@gmail.com. I'll provide you with formulas and methods used to calculate these types.

Regards 

Khawaja Asad Aslam