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Adib Zubaidi Bin RashidMohd Amir Idhzuan Bin JohariHamilah Binti Abd GhaniLai Moon Ting

Precision & AccuracyPrecision & AccuracyDefinitionWays of describing precision

◦Deviation from mean◦Deviation from median◦Range◦Sample standard deviation (s)

Ways of describing accuracy◦Absolute errors◦Relative errors

DefinitionDefinitionPrecision

◦The closeness of results that have been obtained in exactly the same way

◦Generally, the precision of measurement is readily determined by simply repeating the measurement on replicate samples.

Accuracy◦The closeness of measurement to

the true or accepted value and is expressed by the errors.

◦Measures agreement between a result and the accepted value.

- From books of Fundamental of Analytical Chemistry; Skoog, West, Holler & Crouch. Page 93.

Ways of Describing Ways of Describing PrecisionPrecisionDeviation from mean

◦ The mean of two or more measurements is their average value.

◦ Deviation from mean is the differences between the values measured and the mean.

Deviation from median◦ The median is the middle value in a set of data

that have been arranged in numerical order.◦ Deviation from median is the differences

between the values measured and the median.

Range◦ Range is the difference between the highest

and the lowest values.

Example:Example:

From Table 1.1, deviation from mean for each sample are:

di = | xt – x |A = 0.10%B = 0.09%C = 0.01%

di A= 24.39 – 24.28 = 0.10%

From Table 1.1, deviation from median for each sample are:

A = 0.11B = 0.08C = 0.00

Deviation from median for A= 24.39 – 24.28= 0.11

From Table 1.1, range of sample is:

Range = highest value – lowest value = 24.39 – 24.20 = 0.19%

Sample standard deviation (s)

For N (number of measurement) <30 :

For N >30 :

Ways of describing Ways of describing accuracyaccuracyAccuracy are expressed as:Absolute errorRelative error

Absolute error◦Equal to the difference between the

actual reading , xi, and the true (or accepted) value, xt.

EA = xi – xt

Example:From table 1.1, if analysis of chloride is

24.34%, calculate the absolute error.

Absolute error = 24.29% – 24.34% = –0.05%

Relative error◦Describes the error in relation to the

magnitude to the true value◦Normally described in terms of a

percentage of the true value, or in parts per thousand(ppt) of the true value.

◦In percentage:

Er = xi – xt x 100%

xt

◦In parts per thousand(ppt) :

Er = xi – xt x 1000ppt

xt

Example:Calculate the relative error (in percentage and part per thousand), if the absolute error is -0.05% and the accepted value is 24.34%.

Er = xi – xt x 100%

xt

= –0.05 x 100 24.34 = – 0.2%

Er = xi – xt x 1000 ppt

xt

= –0.05 x 1000 24.34 = – 2ppt