Repeatability precision

Control experiment. This is not necessary if the sodium peroxide is known to be chlorine-free. If there is any doubt on this point, the whole operation should be repeated precisely as before, but omitting the organic halogen compound. A small thiocyanate titration value may be found, and this should be deducted from all determinations in which the above quantity of the particular batch of sodium peroxide is used. 

The repeatability precision parameters must be estimated by blind or known replicates. 

Relatively poor repeatability precision (sample inhomogeneity ng range)... 

Repeatability Precision under repeatability conditions, i.e. conditions where independent test results are obtained with the same method on identical test items in the same laboratory by the same operator using the same equipment within short intervals of time. 

It is an indicative valne and should not normally be nsed for decision-making pnrposes. It shonld be established nsing an appropriate measurement standard or sample and should not be determined by extrapolation. The LoQ is calculated as the analyte concentration corresponding to the sample blank value plus 10 standard deviations of the blank measurement. If measurements are made under repeatability conditions, a measure of the repeatability precision at this concentration is also obtained. 

The mean of repeated precise measurement not necessarily is close to the true value (or an accepted estimate of it). In this case the results are precise, but wrong (or better biased). 

This is the Range Chart. It can be nsed only as a repeatability precision check, becanse the target valne for the analysis is not known. This chart only has npper limits. It can be nsed with the range itself (R-chart) or with the percentage difference (R%-chart). 

Precision measures are divided into (1) repeatability precision measures s or SD (sT or SDr) and RSD (RSDr), (2) intralaboratory reproducibility precision or intermediate-precision measures SD and RSD, and (3) interlaboratory reproducibility precision s or SD (sR or SDR) and RSD (RSDr) [66]. 

Minimum of three repeats per concentration level Calculate repeatability precision SDr, RSDr, r = 2.8 x SDr, C, Cl Calculate intermediate precision SD t, RSDbt, r = 2.8 x SD nt, C, Cl Calculate reproducibility precision SDR, RSDr, r = 2.8 x SDr, C, Cl Document in bar chart or control chart... 

Precision is the degree of agreement between a series of measurements obtained from multiple sampling of the same homogeneous sample under prescribed conditions. Precision may be considered at three levels repeatability (precision under the same operating conditions over a short interval of time), intermediate precision (precision within-laboratory variations different days, different analysis, different equipment, etc.), and reproducibility (precision between laboratories, collaborative studies, usually applied to standardization of methodology). 

In chapter 2 I introduced the statistics of repeated measurements. Here I describe how these statistics are incorporated into a quality control program. In a commercial operation it is not always feasible to repeat every analysis enough times to apply t tests and other statistics to the results. However, validation of the method will give an expected repeatability precision (sr), and this can be used to calculate the repeatability limit (r), the difference between duplicate measurements that will only be exceeded 5 times in every 100 measurements. 

Note that repeatability precision (r) is included in the equation for the measurand with a nominal value of 1 and standard uncertainty the Type A standard deviation from repeated measurements. 

The repeatability (precision) of the technique was determined by analyzing 139 urine samples in duplicate. Values ranged from 0.06 to 13.86 mg/24 hr. The mean and standard deviation were 1.597 0.022 mg/24 hr. 

A major improvement came with digital electronics, which allows high-resolulion liming (0.1 second) and exact repeatability. Precise timing of sequential events is essential to multidimensional sample preparation utilizing column separation operators and switching valves. 

The reproducibility standard deviation is typically two to three times as large as that for repeatability. Precision decreases with a decrease in concentration. This dependence has been expressed as RSD = 2° 0 5 exp 108 C), where RSD is expressed as a percentage and C is the concentration of the analyte [38]. For the concentration ranges typically found in pharmaceutical dosage forms (1—10 3), the RSD under conditions of repeatability should be less than 1.0%, and less than 2.0% under conditions of reproducibility [21]. These are similiar to the 1.5% recommendation made for RSD of system repeatability after analyzing a standard solution six times [35]. For method repeatability, which includes sample pretreatment, six replicate assays are made with a representative sample. A RSD no greater than 2% should be obtained. 

Repeatability/ precision Precision under the same operating conditions over a short interval of time. Precision on standards and samples should be evaluated + +... 

Precision. The precision of a method is the degree of scatter of the results and is usually reported as a percentage relative standard deviation. It is often subdivided into repeatability (precision on replicate measurements of the same solution) and reproducibility (precision of the results from measurements of different solutions, i.e. of the complete method). 

Reproducibility is defined as the long-term variability of the measurement process, which may be determined for a method run, within a single laboratory, but on different days. Reproducibility also applies to a method, either run by different operators, different instruments, or a combination of the above. The reproducibility standard deviation is typically twofold to threefold larger than that for repeatability. Precision is often expressed relative to 1 day as intraday (within-day) precision or relative to a period of days, as interday (between days) precision. Reproducibility, in the sense of intralaboratory precision, is related to the procedure being performed at two or more laboratories as in, e.g., a collaborative study. 

The originating laboratory and at least one additional, independent laboratory must participate in the study. Replicate analyses are required to help establish within-laboratory repeatability (precision). PVM methods are announced in Inside Laboratory Management, AOAC s monthly magazine. 

Injection Repeatability. Precision is measured by multiple injections n = 10)22 of the reference standard at the 100% level and indicates the performance of the HPLC instrument using the chromatographic conditions on one particular day and in one lab. The relative standard deviation, RSD(%), as specified here, will determine the lowest variation limit of the analytical results. Injection repeatability indicates the performance of the HPLC instrument using the chromatographic conditions on one particular day and in one lab. 

The separation performance can be of much higher order of magnitude than in HPLC (up to lO theoretical plates), making CE an extremely valuable method for peptide mapping or DNA sequencing. However, small molecules such as amino acids or inorganic ions can be separated as well. The absolute sample amounts which can be injected are low due to the small volume of the capillaries. A major drawback is the lower repeatability (precision) compared to quantitative HPLC. Preparative separations are not possible. 

Repeatability, precision under repeatability conditions, i.e. conditions where independent... 

Repeatability precision—RSD for results on same day with same operator... 

It is the responsibility of any laboratory to demonstrate the validity of the method used. Referring to some third party validation is sufficient if there is a statement that the laboratory can follow the method to ensure the maintenance of the validation. This requires some measure of verification to show, for example, that the analyst in the laboratory can achieve the repeatability precision indicated in the method. 

Repeatability precision is defined as a standard deviation of a mean result determined under conditions in which the same analyst performs a series of independent measurements of a test material with the same measurement system in a short period of time. The repeatability precision represents the smallest variability identified in an experiment. 

The organizing laboratory will collect the results, test for normality of variances and outliers, and then determine repeatability precision and reproducibility precision, either directly or from an analysis of variance (ANOVA) on the data. A one way ANOVA, with the laboratories being the factor studied, will give the repeatability variance as the within groups mean square, and the between groups mean square is the repeatability plus the number of replicates times the laboratory variance. For duplicate determinations these quantities may be obtained directly from the means of the two results and their differences, which will subtract out the laboratory bias. 

When quoting reproducibility data it is important to include a statement of which aspects of the measurement system change. Standard methods that have been subjected to interlaboratory trials (interlaboratory method validation studies) quote the reproducibility as a 95% confidence interval that is, the difference that two laboratories making measurements on portions of the same test material will exceed once in 20 times. The reproducibility precision will include the repeatability precision and components arising from the changes in the system. The relationship between this reproducibility limit, R, and the reproducibility standard deviation Sr is... 

The square root of the Rs is numerically equal to the relative standard deviation of a set of results obtained from 1 g samples by a precise analytical method with negligible This is one of the experimental approaches for determining Rj. Actually, one can estimate R for given w by repeated precise analysis using expression [25]. Once is evaluated for the sample population, the minimum weight, w, needed for a maximum acceptable relative standard... 

While these results do suggest that APPI-MS more closely approaches a universal detector, it should be pointed out that in this test APPI used a dopant (toluene) that was not used in the corresponding APCI tests so that this particular comparison is not straightforward. Also, simple detection is a much easier criterion to satisfy than acceptable accuracy and precision in quantitation. At present the characteristics of APPI with respect to important figures of merit such as repeatability, precision, susceptibility to matrix effects etc. remain unclear. 

Assume that c is the true value of some analytical quantity to be determined. For a given measurement series as in Example 2.1. the error is Lf-cl. We already know that x-> E(x) for A->- 00. In the ideal case, where there is no bias, E(x) = c hence, the error will become zero with increasing sample size. Otherwise, the determination has some bias i (.v) - c >0. and the error can only be reduced to the value of this bias. A statistical test for presence of bias is discussed in Section 3.3.2. The concept of precision refers to the scattering of measurements and is given by the (empirical) standard deviation of a measurement series. As long as precision refers to the results of a. single laboratory, it is identified with the concept of repeatability. Precision of results provided by different laboratories is identified with the concept of reproducibility. For a quantification of both concepts, see Chapter 3.4. 

---