Statistics for Environmental Engineers

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send out more standard specimens and ask the labs to try again. (This may not answer the question. What often happens when labs get feedback from quality control checks is that they improve their performance. This is actually the desired result because the objective is to attain uniformly excellent performance and not to single out poor performers.)


On the other hand, the measurement method might be all right and the true concentration might be higher than 1.2 mg/L. This experiment does not tell us which interpretation is correct. It is not a simple matter to make a standard solution for DO; dissolved oxygen can be consumed in a variety of reactions. Also, its concentration can change upon exposure to air when the specimen bottle is opened in the laboratory. In contrast, a substance like chloride or zinc will not be lost from the standard specimen, so the concentration actually delivered to the chemist who makes the measurements is the same concentration in the specimen that was shipped. In the case of oxygen at low levels, such as 1.2 mg/L, it is not likely that oxygen would be lost from the specimen during handling in the laboratory. If there is a change, the oxygen concentration is more likely to be increased by dissolution of oxygen from the air. We cannot rule out this causing the difference between 1.4 mg/L measured and 1.2 mg/L in the original standard specimens. Nevertheless, the chemists who arranged the test believed they had found a way to prepare stable test specimens, and they were experienced in preparing standards for interlaboratory tests. We have no reason to doubt them. More checking of the laboratories seems a reasonable line of action.

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