Measurement Of Uncertainity

Dear Sir,

What is Measurement Of Uncertaininty ? Kindly give definition and example if any.

Vinod Kumbhar
Lupin Limited,

A measurement tells us about a property of something. It might tell us how heavy an object is,or how hot, or how long it is. A measurement gives a number to that property.

Measurements are always made using an instrument of some kind. Rulers, stopwatches,
weighing scales, and thermometers are all measuring instruments.

The result of a measurement is normally in two parts: a number and a unit of measurement, e.g. ‘How long is it? … 2 metres.’

The uncertainty of a measurement tells us something about its quality. Uncertainty of measurement is the doubt that exists about the result of any measurement. You might think that well-made rulers, clocks and thermometers should be trustworthy, and give the right answers.
But for every measurement - even the most careful - there is always a margin of doubt.

In everyday speech, this might be expressed as ‘give or take’ … e.g. a stick might be two metres long ‘give or take a centimetre’.

Since there is always a margin of doubt about any measurement, we need to ask ‘How big is the margin?’ and ‘How bad is the doubt?’ Thus, two numbers are really needed in order to quantify an uncertainty. One is the width of the margin, or interval. The other is a confidence level, and states how sure we are that the ‘true value’ is within that margin.

For example:
We might say that the length of a certain stick measures 20 centimetres plus or minus
1 centimetre, at the 95 percent confidence level. This result could be written:
20 cm ±1 cm, at a level of confidence of 95%. The statement says that we are 95 percent sure that the stick is between 19centimetres and 21 centimetres long.

Before starting:

Always identify the main error sources, to make sure that they are included in the calculations.

Step Action Measurand:

  1. Specify Measurand (measurand) in (matrix) by (method)

The recommended expanded uncertainty for related analyte

  1. Quantify u(Rw)
    A type uncertainty is used if control and real patient samples are processed by using same protocol

B type uncertainty is used if same steps of the protocol are not used for which are possibly not covered by the control sample

Rw : root mean square of all of the different internal QC samples(lots) CVs

If a control sample is pass through the all steps of the real sample Rw 's enough to calculate of this component but if real samples need to be processed before being analysed every source of uncertainty components should be taken in to account. This two types of uncertainty components are called A and B types uncertainty respectively.

  1. Quantify method and laboratory bias
    Quantifying the laboratory bias is based onthe data obtained from external QC reports. Root mean square of the RMS(bias) and u(cref) is called u(bias).

RMS(bias) is the root mean square of the last three years inaccuracy scores obtained from the end of cycle reports of external QC data.

u(cref) is basicly calculated by using the lab quantity which shows participated laboratories to the concerning external QC programme and bias of method in which the related result is assessed.

  1. Converting all components of the uncertainty to a standard uncertainty u(x)

  2. Calculate combined standard uncertainty, uc

A combined standart uncertainty is calculated by using the root mean square of all standart uncertainty components.

  1. Calculate expanded uncertainty, U = 2 x uc

Expanded uncertainty is calculated by multiplying 2 of the combined uncertainty.
Two (1,96 in certain) is for the 95% confidence interval.