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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