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: d i = | x t – x | A = 0.10% B = 0.09% C = 0.01% d i A= – = 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 = – = 0.11

From Table 1.1, range of sample is: Range = highest value – lowest value = – = 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, x i, and the true (or accepted) value, x t. E A = x i – x t 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: E r = x i – x t x 100% x t ◦ In parts per thousand(ppt) : E r = x i – x t x 1000ppt x t

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%. E r = x i – x t x 100% x t = –0.05 x = – 0.2% E r = x i – x t x 1000 ppt x t = –0.05 x = – 2ppt