Clarification on Basic Principle of Uncertainty (Type A & B)

N

neoth

#1
Hi All,

Recently I am test on this basic uncertainty. And I don't understand it quite well, so I hope to seek some clarifications from experts here.:bigwave:

From what I know, uncertainty can be classified into type A & B.

Type A consist of:
-the resolution of the unit under test(UUT)
-the repeatabilty of the uut
-the hystersis
-drift.

Type B consist of:
-manufacture specifications.

Am I correct? Did I left out anything?
So what do I deal with each individual values , do I divided them by sqrt3?

Someone please enlighten me. Thanks in advance.:thanx:
 
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BradM

Staff member
Admin
#2
Hello there! You ask good questions. Let me encourage you do take some time and poke around here at the Cove. There are lots of resources for you, especially in the attachment section. Notice the green box at the top of the page that says post attachment list. Click on it, and use uncertainty as a key word.

Here is one good attachment Randy Stewart has provided:

http://elsmar.com/Forums/showpost.php?p=78162&postcount=36

Here is a good thread on it:

http://elsmar.com/Forums/showthread.php?t=5689


Within that thread, this post has some helpful attachments also:
http://elsmar.com/Forums/showpost.php?p=72085&postcount=56

Hope that helps.
 
N

neoth

#3
Hi bradM,

tks for all the link and attachment. but they are too complicated for me, :confused:...
find it hard to understand all...
 

BradM

Staff member
Admin
#4
Hi bradM,

tks for all the link and attachment. but they are too complicated for me, :confused:...
find it hard to understand all...
Basically, Type A would be based on statistical methods.

Type B is "scientific judgment" of the data. So anything you collect from specifications, other calibration data, uncertainty of standard, etc., would be Type B.

Anything that you would calculate using statistics is Type A.

Hope that helps.
 
N

neoth

#5
hi,

just wondering what kind of distribution to use?
should i use a normal, rectangular,trigular or u-shape?

regards
 
J

Joe222

#6
The distribution used will vary by what the error source is. Uncertainty taken from a calibration certificate will be a normal distribution (devide by 2 if it has a coverage factor of 2). U-Shaped is used when the error source is related to temperature. Rectangular is used for a wide variety of errors such as resolution.
 
S

stefanhg

#7
JCGM 100:2008, GUM 1995 with minor corrections
Evaluation of measurement data — Guide to the expression of uncertainty in measurement

Type A evaluation (of uncertainty)
method of evaluation of uncertainty by the statistical analysis of series of observations

Type B evaluation (of uncertainty)
method of evaluation of uncertainty by means other than the statistical analysis of series of observations

These categories apply to uncertainty and are not substitutes for the words “random” and “systematic”. The uncertainty of a correction for a known systematic effect may in some cases be obtained by a Type A evaluation while in other cases by a Type B evaluation, as may the uncertainty characterizing a random effect.

The purpose of the Type A and Type B classification is to indicate the two different ways of evaluating uncertainty components and is for convenience of discussion only; the classification is not meant to indicate that there is any difference in the nature of the components resulting from the two types of evaluation. Both types of evaluation are based on probability distributions, and the uncertainty components resulting from either type are quantified by variances or standard deviations.

The estimated variance u^2 characterizing an uncertainty component obtained from a Type A evaluation is calculated from series of repeated observations and is the familiar statistically estimated variance s^2 . The estimated standard deviation u, the positive square root of u^2, is thus u = s and for convenience is sometimes called a Type A standard uncertainty. For an uncertainty component obtained from a Type B evaluation, the estimated variance u^2 is evaluated using available knowledge, and the estimated standard deviation u is sometimes called a Type B standard uncertainty.

Thus a Type A standard uncertainty is obtained from a probability density function derived from an observed frequency distribution , while a Type B standard uncertainty is obtained from an assumed probability density function based on the degree of belief that an event will occur [often called subjective probability ]. Both approaches employ recognized interpretations of probability.

A Type B evaluation of an uncertainty component is usually based on a pool of comparatively reliable information.
For an estimate xi of an input quantity Xi that has not been obtained from repeated observations, the associated estimated variance u^2(xi) or the standard uncertainty u(xi) is evaluated by scientific judgement based on all of the available information on the possible variability of Xi . The pool of information may include
- previous measurement data;
- experience with or general knowledge of the behaviour and properties of relevant materials and instruments;
- manufacturer's specifications;
- data provided in calibration and other certificates;
- uncertainties assigned to reference data taken from handbooks.
For convenience, u^2(xi) and u(xi) evaluated in this way are sometimes called a Type B variance and a Type B standard uncertainty, respectively.
 
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