Control Charts for Environmental Data
The ability to accurately monitor the level of toxic pollutants present in the environment has become a problem of great concern to government, business, the public, and to researchers. Recently, the EPA suggested using control charts to monitor toxic pollutants in the environment. However, the reporting procedures used are complicated by at least three factors. First, the sample sizes used to characterize the mean level of a toxic contaminant are typically small. The primary reason for taking small samples is simply a matter of technology and economics, i.e., measurements of toxic contaminants usually involve expensive laboratory tests. As a result, the estimated mean value of a given pollutant is based only on a small sample of observations. Even so, when the underlying data from which the sample has been drawn were to follow a normal distribution, the usual small-sample confidence intervals are often very wide.
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