Monitoring statistics discussion

Historical DataBase Subsystem We have discussed the use of on-hne databases. An historical database is built similar to an on-line database. Unlike their on-line counterparts, the information stored in a historical database is not normally accessed directly by other subsystems for process control and monitoring. Periodic reports and longterm trends are generated based on the archived data. The reports are often used for long-term planning and system performance evaluations such as statistical process (quality) control. The trends may be used to detect process drifts or to compare process variations at different times. 

A radioactive isotope may be unstable, but it is impossible to predict when a certain atom will decay. However, if we have a statistically large enough sample, some trends become obvious. The radioactive decay follows first-order kinetics (see Chapter 13 for a more in-depth discussion of first-order reactions). If we monitor the number of radioactive atoms in a sample, we observe that it takes a certain amount of time for half the sample to decay it takes the same amount of time for half the remaining sample to decay, and so on. The amount of time it takes for half the sample to decay is the half-life of the isotope and has the symbol t1/2. The table below shows the percentage of the radioactive isotope remaining versus half-life. 

Clearly, a balanced program that gives careful consideration to the limitations of using CROs — to run the studies, provide the statistical analyses, monitor the processes, and even, coordinate the studies performed outside the United States — needs to be evaluated against the more traditional approaches to drug development. The CRO in clinical research is discussed in detail in Chapter 21. 

The specific problems discussed above emphasize that environmental chemistry poses considerably harder problems to the chemometrician than straight analytical chemistry [BRERETON, 1995]. The current state of environmental analysis often involves empirical planning of experiments and monitoring, as well as expensive and time-consuming analysis, with the result that only simple statistics are applied to the data obtained. In practice, simple comparison of data averaged in time or space with legally fixed thresholds or limits is often performed at the end of the environmental analysis process. Because environmental data contains so much information, chemometric methods should be used to extract the latent information from these data. 

It must be apparent from the above discussion that a number of variables other than concentration affect the intensities of mineral patterns as observed on a diffractogram. Inasmuch as they cannot be monitored and compensated for mathematically, these variations must be reflected in the error of determination. This would be true regardless of the quantification procedure employed. The conclusion, therefore, is that the inherent variability in composition and/or crystallinity that exists within the major mineral components of the low temperature ashes of coal will be reflected in the statistical error of determination and that error will be of sufficient magnitude to preclude the use of the term "quantitative" to describe the procedure. Therefore, any procedure using x-ray diffraction to determine the minerals in coal must be considered semi-quantitative at best. 

The book follows a rational presentation structure, starting with the fundamentals of univariate statistical techniques and a discussion on the implementation issues in Chapter 2. After stating the limitations of univariate techniques, Chapter 3 focuses on a number of multivariate statistical techniques that permit the evaluation of process performance and provide diagnostic insight. To exploit the information content of process measurements even further. Chapter 4 introduces several modeling strategies that are based on the utilization of input-output process data. Chapter 5 provides statistical process monitoring techniques for continuous processes and three case studies that demonstrate the techniques. 

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