Reliability of measurement

Suppose the measurements are repeated with a caliper whose vernier scale permits careful measurement to the nearest 0.01 cm and estimation to the 

In talking about the results of measurements, we distinguish between the accuracy and the precision of the results. The accuracy of a series of measurements tells how closely the average of the results agrees with the true value of the quantity that is measured. The precision of a series of measurements tells how nearly the repeated measurements yield the same result. For example, suppose that the markings on a centimeter scale are placed too far apart (as if the scale has been stretched). In this case, the results obtained in a series of measurements of the same object might be quite precise (different measurements would yield nearly the same answer), but they would be inaccurate (the average result would be far from the true value). 

Measurements commonly involve systematic errors. These are errors that are reproducibly introduced in each measurement because of the construction, use, or calibration of the equipment (as in the case of the stretched scale). The precision of the results may give the illusion of accuracy in such cases. For this reason, it is desirable to make a measurement by various entirely different methods. If the results still show high precision (close agreement with one another), then it is unlikely that systematic errors exist. The accuracy of the measurement can also be tested by using the same measurement methods on a standard sample whose value has been certified by some reliable institution, such as the National Bureau of Standards. 

Measurements also commonly involve random errors. These are errors whose size and direction differ from measurement to measurement that is, they are unpredictable and unreproducible. They are commonly associated with the limited sensitivity of instruments, the quality of the scales being read, the degree of control over the environment (temperature, vibration, humidity, and so on), or human frailties (limitations of eyesight, hearing, judgment, and so on). We shall say much more about random error later in this chapter. 

All digits of a number that are reasonably reliable are known as significant figures. The number 1.79 has three significant figures 1, 7, and 9. The number 1.794 has four significant figures. 

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In developing the procedure of testing the features of NDT technological process must be taken into account, particularly, availability of the object for testing, strict characteristics of NDT devices being used and other factors which influence on the accuracy and reliability of measurements. 

NDT equipment widely used for definition of safety and quality of produets must ensure uniformity and reliability of measurements realised with them. 

Random deviations (errors) of repeated measurements manifest themselves as a distribution of the results around the mean of the sample where the variation is randomly distributed to higher and lower values. The expected mean of all the deviations within a measuring series is zero. Random deviations characterize the reliability of measurements and therefore their precision. They are estimated from the results of replicates. If relevant, it is distinguished in repeatability and reproducibility (see Sect. 7.1)... 

The chief objective of this section is to provide a perspective concerning the reliability of atmospheric-oxidant data. The expected performance of atmospheric-oxidant monitors is given in Table 6-7. To judge the reliability of measurements, information about the following five factors is required ... 

During the analysis process there is the stage of measuring the signal attributed to the analyte. The analytical chemist must prove whether this signal corresponds to the analyte or whether part of it is to be attributed to interfering impurities. This is the confirmation of identity. Selectivity is a measure that assesses the reliability of measurements in the presence of interferences. 

The real and imaginary parts of the complex refractive index satisfy Kramers-Kronig relations sometimes this can be used to assess the reliability of measured optical constants. N(oj) satisfies the same crossing condition as X(w) N (u) = N( — u). However, it does not vanish in the limit of indefinitely large frequency lim JV(co) = 1. But this is a small hurdle, which can be surmounted readily enough by minor fiddling with JV(co) the quantity jV(co) — 1 has the desired asymptotic behavior. If we now assume that 7V( ) is analytic in the top half of the complex [Pg.28]

The average is 10.320 m, with an average deviation of 0.0048 m. It is proper to write the average as 10.320 m, because the deviation affects digits in only the third decimal place. It is not correct to give the length as 10.32 m this implies that the measurement is uncertain in the second decimal place. Average deviation is one of the simplest measures of reliability of measurements (the spread of experimental values), but a better estimate of reliability can be made with standard deviation. 

The reliability of measurements plays a pivotal role in food and agricultural areas, particularly in the case of undesirable toxic compounds such as mycotoxins. Quality-control principles for mycotoxin analysis are common to other trace analyses, so good laboratory practices, such as EN 4500, represent the heart of quality assurance requirements. Harmonized Guidelines for Internal Quality Control in Analytical Chemistry Laboratories, published by IUPAC (23), also presents valuable guidelines for the determination of mycotoxins. 

The Reliability of Measurements. The Analysis of Data. The Application of Statistical Tests. Limits of Detection. Quality Control Charts. Standardization of Analytical Methods. 

In many cases the reliability of measurement is based on reliability of reference materials. Therefore the important role of reference materials is well recognized. 

Communautaire. Accreditation aims at the reliability of measurement results and enables laboratories to demonstrate technical competence to their customers. Metrological concepts in food science have become a widely discussed topic over the last few years. Representing the top of the international measurement infrastructure, the National Metrology Institutes (NMIs) have the mandate to disseminate the best practice and measurement capabilities to the beld laboratories (FLs) in their countries. The European and international measurement infrastructure needs high-quality tools to assess the reliability and comparability of measurements results in view of the implementation of EU and international policies regarding internal market activities as well as health and consumer protection issues. 

Cruz SL, Salazar LA, Villarreal JE (1991) A methodological basis for improving the reliability of measurements of opiate abstinence responses in the guinea pig ileum made dependent in vitro. J Pharm Meth 25 329-342... 

Collen FM, Wade DT, Bradsbaw CM (1990). Mobility after stroke reliability of measures of impairment and disability. International Disability Studies 12 6-9... 

The reliability of measurements. The arrptysis of data. The application of statistical tests. Limits of detection. Quality control charts. Standardization of analytical methods. Chcmometrics. 

Spencer CA, Takeucho M, Kazarosyn M, et al. Interlaboratory/intermethod differences in functional sensitivity of immunometric assays for thyrotropin (TSH) Impact on reliability of measurement of subnormal concentration. CHn Chem 1995 41 367-74. 

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