UCL GEOL0076 Environmental Geosciences Lecture Notes PDF

GEOL0076: Introduction to Environmental Geosciences Lecture 3: The concept of data quality 1  Lecture 3: The concept of data quality Delivery plan ❖ Concepts of replicate measurements, and Data quality. ❖ Precision & acc...

GEOL0076: Introduction to Environmental Geosciences Lecture 3: The concept of data quality 1  Lecture 3: The concept of data quality Delivery plan ❖ Concepts of replicate measurements, and Data quality. ❖ Precision & accuracy of measurements Intended learning outcome At the end of this session, you should be able to : ❖ Explain the concepts of data quality. ❖ use precision & accuracy to evaluate the data quality of measurements 2  Data quality Data quality is a measure of how reliable a set of data is with respect to the expected information. ❖ It is a measure of how “fit” Data quality the data is to meet the specific needs, or High or Low or ❖ How well suited a set of Good poor data is for the intended purpose 3 4  What to do to ensure Data Quality Accuracy Include quality control standard, & evaluate the accuracy Consistency Replicate the measurements & evaluate the Precision Integrity Record at the time of measurement, sign it off Timeless Redo the measurements after some period of time, e.g 1, 3, 6, 12 months, etc Validity A combination of accuracy, consistency, integrity, & timelessness Completeness A complete information for the intended purpose 5  Replicate measurement ❖ Scientific measurements are usually encouraged to be in replicates ❖ This helps to identify misleading data, and ensures data reliability ❖ All your laboratory measurements should be carried out in triplicates (taken 3 times) 6  Accuracy & Precision ❖ One way of evaluating data quality is by precision & accuracy ❖ Taking replicate measurements helps to evaluate precision ❖ For accuracy, quality control standard (a substance with known value of the measured parameter), is required ❖ It is also a common practice to use quality standards, to check the performance of a method, after certain intervals of the measurements ( after every 6 samples, etc.) 7 Precision ❖ Precision: if you take a measurement on a particular parameter for more than once, there is a likelihood that the values may not be exactly the same. ❖ Precision refers to how close each of these measurements is to each other. ❖ It gives a measure of repeatability /reliability of the observed data Accuracy ❖ Accuracy refers to the closeness of a measured value to the true (known value) 8  Evaluation of precision ❖ The standard deviation or % Relative standard deviation (% RSD) is most commonly used to evaluate precision in scientific measurements ❖ Relative standard deviation (% RSD) = SD X 100 Mean  Relation between% RSD and precision If your % RSD is small say 2.5% or less, the result tells you that your standard deviation is a small % of the mean In other words, the data is tightly clustered around the mean. On the other hand, if % RSD is large, say, 55%–this would indicate your data is more spread out. The RSD provides an idea on how precise your data is in an experiment. The more precise your data, the smaller the % RSD. We will do some examples on excel spreadsheet  EAXMPLE 1: Using the pH values of some soil samples Below are the pH values in triplicate for some soil samples S/N Soil samples Test 1 Test 2 Test 3 1Contaminated soil 5.6 5.5 5.7 2Uncontaminated soil 6.7 6.8 6.7 Certified Reference 3 material soil (Quality 7.1 7 7.1 control, PH= 7) Evaluate the precision and accuracy of the measurement 11  Enter the values onto an Excel spreadsheet, against the respective soil samples S/N Soil samples Test 1 Test 2 Test 3 1Contaminated soil 5.6 5.5 5.7 2Uncontaminated soil 6.7 6.8 6.7 Certified Reference 3 material soil (Quality 7.1 7 7.1 control) 12 Precision continues Using the excel spread sheet, evaluate the mean, Standard deviation (SD), and %RSD (a measure of precision) To do this create a column to evaluate the mean, SD, and %RSD, along the table as shown Mean S/N Soil samples Test 1 Test 2 Test 3 SD % RSD pH 1Contaminated soil 5.6 5.5 5.7 Uncontaminated 2 6.7 6.8 6.7 soil Certified 3 Reference 7.1 7 7.1 material soil 13 Precision continues To calculate the mean, choose the cell you want to enter the calculation of the mean Type in =, followed by typing the word ‘av’ This brings up the ‘averaging options’, 14 Precision continues From the ‘averaging options’, select the option ‘average’, then select the cells that you want the averaging to de done Then press enter, this does the calculation for the mean for that cell 15  Once the averaging has been done for one cell, hold the small sign at the end of the box and drag it down That does the averaging for the rest of the samples 16 Precision continues To calculate the standard deviation (SD), type in = in the cell where you want to do the calculation, followed by typing ‘st’. This brings up the options for standard deviation, from where you select ‘STDEV.S’ 17 Precision continues The select the cells which you want the SD to be done for, and press enter This calculate the standard deviation for the set of parameters which you have selected 18 Precision continues Once the SD has been done for one cell, hold the small sign at the end of the box and drag it down That does the SD for the rest of the samples 19 Precision continues To calculate the % RSD (which is a measure of precision) The formula is [(SD/Mean) x 100] So type in, = in the cell where you want to do the calculation for %RSD This is followed by opening up a bracket, then click the cell containing the SD value, type the sign of division (/)and click on the cell containing the mean. Close the bracket and type in * 100, then enter 20 Precision continues This calculate the %RSD for the cell. Hold down the small sign at the corner of the cell and drag it down to get the calculations for the other cells 21 Accuracy 22  Accuracy Accuracy is often reported in % In the case of the certified reference soil, you will report the % recovery of the certified values Measured value % Accuracy or Recovery = x 100 Certified value  Accuracy How do you know the concentration you measure is correct? Use Certified Reference Materials (CRMs) or quality standards Evaluate % accuracy (% Recovery) Measured value % Accuracy = x 100 True/Certified value E.g if the average measured pH for PH check solution was 6.98, against the true value of 7.0 6.98 % accuracy for Ca = x 100 = 99.71% 7.00 You can also evaluate % error in the estimation as 100-99.71% = 0.002%  Accuracy Another way is to calculate the error first, Then subtract the % error from 100% to have your % accuracy Actual value −Measured value % error = x 100 Actual value E.g. if the measure PH of cert Ref soil =6.99 And the Actual/True value (provided) = 7.01 (7.01−6.99) % Error for pH = x 100 = 0.29% 7.01 % Accuracy = 100% - % Error = 100 - 0.29% = 99.71%  Class activity: Evaluate the precision and accuracy of the following measurements: Evaluate the data quality of the measurement Mean S/N Soil samples Test 1 Test 2 Test 3 SD % RSD pH 1Water sample 1 6.6 6.5 6.8 2Water sample 2 6.7 6.9 6.6 Quality control 3 6.9 7.0 7.02 (True value =7.0) Now: Evaluate the data quality of the measurement of your measurements during the last lab activities Question Time 27