Repeatability of measurement

The standard deviation as defined relates to the repeatability of measurements on the same sample. When many samples are taken from a large population, sampling variability and population variability terms have to be added to Eq. (1.6) and the interpretation will reflect this. 

Another major change was the shift from extensive use of field laboratory exploration techniques to the laboratory techniques hke ICP-AES and INAA. These produce a higher quality data than had resulted from the dc arc and other field techniques, with respect to both repeatability of measurement and improved detection limits. The metrology laboratory certifications for As and Hg in soils and sediments as key environmental toxins provided strong support to mineral exploration programs. 

Densities were measured using a Paar DMA 60 meter equipped with DMA 512 and DMA 601 HP external cells. Values in the 50-150°C range were interpolated from measured data (3-5 points) values above 150°C were extrapolated and are less accurate. Interfacial tension measurements at the minimum density difference encountered (0.05 g/cm3) could be in error by as much as 10%, which is within the repeatability of measurements with heavy crude oil samples (see below). 

Modifications of the conventional spinning drop tensiometer were required for operating at temperatures up to 200°C. Measurements carried out with heavy oil samples required the use of D20 instead of H20 to maintain a sufficient density difference between oil and water. For accurate measurements, considerable care must be used to ensure that heavy oil drops do not lag behind the rotation of the capillary tube in the tensiometer. Also, repeatability of measurements conducted with chemically ill-defined substances may be hampered by the inhomogeneity of the oil drops. 

Checking for wavelength repeatability by using one or more suitable standards (e.g. polystyrene or rare-earth oxides). The repeatability of measurements should be consistent with the spectrophotometer specification. 

The participants reported high precision of results (intralaboratory repeatability of measurements). The and M data obtained in particular laboratories scattered less than 3%, often even in the range of 1%. This is an excellent result, indeed. Unfortunately, the high repeatability of measurements may lead to a notion that the results are also exact. 

Repeatability Obtain as standard deviation of repeated weighings or assume part of repeatability of measurement = s. = 0.2 mg... 

Based on an interlaboratory study, the AOCS estimates that the repeatability of measurements should be 1.3% and the between-laboratory variability 3.3% (Firestone, 1998). 

Instruments which can monitor the important process variables during plant operation must be specified. These instruments must be capable of measuring the variables and should have an acceptable accuracy and repeatability of measurement, usually the latter attribute is more important than the former on chemical plant measurements. The instruments may be used for manual measurements or included in automatic control loops. Automatic alarms may also be required to indicate deviations outside acceptable limits. If possible, direct measurement of the process variable should be made, however it is often easier to measure a dependent variable, e.g. temperature measured as an indication of composition for distillation column top product. 

For physical and chemical measurements it is essential that measurements are referenced to standards accepted by all the laboratories undertaking a particular type of measurement. In particular, the use of chemical analytical procedures validated through their application to certified reference materials (CRM) (26) is highly recommended. It is to be noted that a number of CRMs prepared with Antarctic matrices are already available, i.e., marine sediment and krill, or in preparation (26-29). These and other CRMs should be also used routinely by the participating laboratories to assure a periodical assessment of accuracy and repeatability of measurements. These laboratories should also undertake regular intercalibration studies. It is possible that in the first application of these exercises systematic errors will be found, but better results are expected in subsequent rounds together with a general improvement in the performance of laboratories and data quality. 

Internal markers or standard proteins have most often been used by researchers to assure themselves that separations by electrophoresis are consistent and reproducible (21). In addition, charge markers In Isoelectric focusing are also used, although standard preparations of these materials are less reliable, especially under denaturing conditions. Other Internal standards that are often reported are materials for assuring radlolabel activity, enzyme activity, or other materials to monitor repeatability of measurements. In all cases, the stability of the standard material Is the key to long-term quality control. Large, batches of standards that are reproducible from lot to lot would also be useful. 

More recently data reportability has been linked to the repeatability of measurement of samples after a more or lass extensive sample preparation and analysis process. Many analytical chemists currently consider it improper to report measurements observed to be below their Method Detection Limit (MDL). They fail to recognize that the measurement was in fact above the IDL and is therefore a real result. 

Precision characterizes the repeatability of measurements. For n independent random measurements, it can be described by using the standard deviation as the dispersion measure. In the concentration domain, x, is calculated according to Eq. (2.4) by... 

Periodic repeats of measurements and computations were made. 

Table 3. Repeatability of measurements, results of duplicate measurements.

It is a good practice to conduct a short calibration phase prior to the production phase to make sure the waves were created correctly and are being simulated accurately. It is also a good practice to repeat the individual wave cases, if possible, to ensure that the recorded values are correct and give some confidence in the repeatability of measured values from a statistic standpoint. 

In methods like TLC, validation must be performed in more levels. In the basic level, a scanner must be checked and calibrated according to the manufacturer s specifications. In the second level, it must be validated with a special test plate. This must be done at least once in a year, in order to obtain data about mechanical robustness, repeatability of measurements, monochromator accuracy, baseline noise, and the signal-to-noise ratio. This test plate is prepared by a vendor, and its purpose is detection of any possible malfunction. Too often, use of this informative but time-consuming and complicated test operation is questionable, especially because people in the laboratory are usually not able to repair a scanner, so a service-call is required. In order to know the actual quality of a scanner, a user must prepare a simpler procedure. This procedure can use a part of a vendor test plate, but it is better for each user to prepare an additional in-house test plate. The purpose of such a test is to check the quality of a scanner under working conditions on a daily basis. For accurate results, a third level may be introduced. The best way is to make a system-suitability test on each plate, selecting one spot on a plate, and measuring it with the working paramaters. 

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