Interpretation of Measured Variance

The total measured variance for a set of samples taken from bulk (population) is summation of contributions from various sources namely. 

The sampling and analytical variances must be determined from independent measurements. It is well established that once the analytical uncertainty (standard deviation) is reduced to a third or less of the sampling uncertainty, further reduction in analytical uncertainty is of little importance. Therefore, if the uncertainty in sampling is very large, it may be beneficial to opt for an analytical method that is rapid even though it might have lower precision. This will permit more samples to be analyzed, thereby resulting in a better estimate of the mean value. 

To evaluate the state of mixedness of a mixture, a representative sample must be retrieved and analyzed. The result of this analysis combines errors due to sampling, analytical method, and uncertainty due to state of the mixture. Following certain guidelines and good practices can minimize the error due to sampling and analysis. The subject of sampling is very complex and detailed treatment can be found in the literature (Hersey, 1970 Kristensen, 1973 Sommer, 1986 Muzzio et ah, 1997). The objective of this section is to outline important issues and provide practical guidance. 

Some common questions pertaining to sampling encountered during mixer selection and performance evaluation are  

1 Sampling Location. The selection of sampling locations depends on the objective of the study and mode of mixer operation. The objective of the mixing study could be (1) selection of a mixer for a given process, or (2) 

---