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GROUP 1
Adib Zubaidi Bin Rashid
Mohd Amir Idhzuan Bin Johari
Hamilah Binti Abd Ghani
Lai Moon Ting
Precision & Accuracy
Definition
 Ways of describing precision

◦
◦
◦
◦

Deviation from mean
Deviation from median
Range
Sample standard deviation (s)
Ways of describing accuracy
◦ Absolute errors
◦ Relative errors
Definition

Precision
◦ 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 Precision

Deviation 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:
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 = 0.11
B = 0.08
C = 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 accuracy
Accuracy are expressed as:
 Absolute error
 Relative 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
xt
x 100%
◦ In parts per thousand(ppt) :
Er = xi – xt
xt
x 1000ppt
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