Accuracy estimates measurement

The molten salt standard program was initiated at Rensselaer Polytechnic Institute (RPI) in 1973 because of difficulties being encountered with accuracy estimates in the NBS-NSRDS molten salt series. The density, surface tension, electrical conductivity, and viscosity of KNO3 and NaCl were measured by seven laboratories over the world using samples distributed by RPI. The data from these round-robin measurements of raw properties were submitted to RPI for evaluation. Their recommendations are summarized in Table 2. 

Accuracy is a measure of how close a measurement is to the "true" value. While it is impossible to determine absolute accuracy it is possible to obtain an accuracy estimate using several techniques. 

Certified Reference Materials. Certified Reference Materials are materials whose properties have been guaranteed or certified by recognized bodies. The certified analyses of these materials can be used as an estimate of the "true" value for assessment of accuracy. The United States National Bureau of Standards (NBS) provides an inventory of various materials whose compositions (and properties) have been measured using definitive and reference methods. These materials, Standard Reference Materials (SRM s), when used in conjunction with reference methods, i.e., one of demonstrated accuracy, make it possible to transfer accuracy between measurement protocols. 

When estimating measurement results, the important thing is not only to know its accuracy but also the measurement confidence. The degree of measurement the confidence is estimated from confidence interval as defined by the level of significance. Let X denote the actual measurement value and AX the error in measuring... 

Absorption, HSC>3, 180 Accommodation coefficients calculations, 111 measurements, 104-105 ozone, 109-116 sulfur dioxide, 109-116,154 Accuracy, estimation for an instrument, 188 Acetate ions, introduction into precipitation, 219-227 Acetic acid... 

The available sample size for kidney, brain, liver, and gut varied from 20 to 100 grams. Replicate assays of these samples did not agree and recovery studies indicated that these assays were not quantitative. Assays of one whole kidney showed significant differences between the concentrations of DDT plus DDE in the cortex and the medulla. The precision and the accuracy, as measured by recovery studies of replicate samples from homogenized portions of either the cortex or the medulla, were within the limits considered acceptable for this method. With the realization that it was impossible to obtain reliable data on the samples as they had been collected, an arithmetic average of all replicate assays for the organs from four cadavers was used to estimate the total body burden. Cadavers 3, 10, 14, and 18 were studied. 

However, despite Q3 being the most popular and widespread measure it suffers serious problems in terms of providing a reliable and significant accuracy estimate. The main Q3 drawback is that it does not take in account under- and over-predictions failing to capture the real significance of the results. For example, if we predict all the residues as being coil in the test database, an average Q3 value of 48.19% is obtained but correlation coefficients and information values will be null. 

Estimates 21 Estimates of diagnostic accuracy and measures of statistical uncertainly (e.g. 95% confidence intervals). ... 

Accuracy is a measure of how close to truth a method is in its measurement of a product parameter. In statistical terms, accuracy measures the bias of the method relative to a standard. As accuracy is a relative measurement, we need a definition of true or expected value. Often, there is no gold standard or independent measurement of the product parameter. Then, it may be appropriate to use a historical measurement of the same sample or a within-method control for comparison. This must be accounted for in the design of experiments to be conducted for the validation and spelled out in the protocol. Accuracy is measured by the observed value of the method relative to an expected value for that observation. Accuracy in percent can be calculated as ratio of observed to expected results or as a bias of the ratio of the difference between observed and expected to the expected result. For example, suppose that a standard one-pound brick of gold is measured on a scale 10 times and the average of these 10 weights is 9.99 lbs. Then calculating accuracy as a ratio, the accuracy of the scale can be estimated at (9.99/10) x 100% = 99.90%. Calculating the accuracy as a bias then [(9.99 - 10)/10] X 100% =-0.10% is the estimated bias. In the first approach ideal accuracy is 100%, and in the second calculation ideal bias is 0%. 

The calculated and experimental values in Table 6.10 show quite reasonable agreement. The accuracy of these comparisons is limited by use of limiting equivalent conductances instead of the equivalent conductances at the concentrations actually used, imprecise temperature control, and somewhat limited accuracy in measuring the cell constant. Nevertheless, calculations from a table of equivalent conductances provide a useful estimation of expected experimental results. 

For estimation of the performance of an algorithm or a recognition function we will use several quality measures [42, 46, 47]. True positives (TP) is the number of correctly predicted and false positives (Fp) is the number of falsely predicted authentic sites. True negatives (Tn) is the number of correctly predicted and false negative (FN) is the number of falsely predicted non-sites. Sensitivity (S ) measures the fraction of the true examples that are correctly predicted Sn = Tp/TP + FN. Specificity (Sp) measures the fraction of the predicted examples that are correct Sp = Tp/Tp + Fp. Only consideration of both Sn and Sp values makes sense when we aim at providing accuracy information. If we want to concentrate on a single value for accuracy estimation the average of the correctly predicted number of sites and non-sites AC = 0.5 (TP + TN) is a suitable measure. However, this... 

The rms fit values of Table 7 are small enough compared to our estimated +100 cm 1 accuracy of measuring tJmax to call fmr linear with y. Although we do not believe it can be stated that solvent donicity causes the changes in vmaK that are observed, use of the three-parameter Equation (17) produces A values that we argue are close to 7.v because of their dependence upon structure of the IV compound. As shown in Scheme 7, the z.v values obtained in Table 7 for similar structural units are... 

The mass flux of the dispersed phase is in fact an essential measurement quantity in many experimental investigations. The accuracy of mass flux measurements will depend not only on the instrumentation, but also on the flow field and the size distribution of the dispersed phase, so that a general accuracy estimate is not feasible. In simple spray flows however, an accuracy of 10% on the local mass flux can be expected (Sommerfeld and Qiu 1995, Mundo 1996). 

Control of a number of physicochemical factors is critical to achieving maximal precision and accuracy in measured values. These include the choice of e>q)eri-mental method, pH meter calibration, temperature control, solvent composition, ionic strength, absence of atmospheric CO2 contamination, estimation methods for activity coefficients, and chemical stability. 

Reproducibility. The overall accuracy of the thermal conductivity apparatus is estimated to be 10%. This estimate is based on the accuracy of measuring the volume of boil-off gases, controlling the gas pressures above the crypgenic liquids in the measuring and guard vessels, the effect of atmospheric pressure changes on the boil-off rates, and the temperature measurement of the cold and warm surfaces. The estimate is supported by the electrical heater calibration tests. 

The accuracy defined as the deviation of deduced concentrations from the respective true values depends first on the accuracy obtained in the calibration of the measuring configuration. For thin samples, a careful calibration and evaluation procedure can result in the estimated accuracy of 2-3% between 22 < Z < 30 and 5% between 11 < Z < 20. However, it should be kept in mind that the analytical accuracy in measurements performed on actual thin samples may be degraded by effects pertinent to the applied sampling technique. As described in Sect. 33.1.4.2 special care should be taken with regard to stopping, absorption, and matrix effects in thick sample cases. 

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