Statistics for Environmental Engineers

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Graphical methods are obviously useful for both initial and exploratory data analyses, but they also serve us well in the final analysis. “A picture is worth a thousand words” is a cliche, but still powerfully true. The right graph may reveal all that is important. If it only tells part of the story, that is the part that is most likely to be remembered.

Tables of numbers camouflage the interesting features of data. The human mind, which is remarkably well adapted to so many and varied tasks, is simply not capable of extracting useful information from tabulated figures. Putting these same numbers in appropriate graphical form completely changes the situation. The informed human mind can then operate efficiently with these graphs as inputs. In short, suitable graphs of data and the human mind are an effective combination; endless tables of data and the mind are not.

It is extremely important that plots be kept current because the first purpose of keeping these plots is to help monitor and, if necessary, to troubleshoot difficulties as they arise. The plots do not have to be beautiful, or computer drafted, to be useful. Make simple plots by hand as the data become available. If the plots are made at some future date to provide a record of what happened in the distant past, it will be too late to take appropriate action to improve performance. The second purpose is to have an accurate record of what has happened in the past, especially if the salient information is in such a form that it is easily communicated and readily understood. If they are kept up-to-date and used for the first purpose, they can also be used for the second. On the other hand, if they are not kept up-to-date, they may be useful for the second purpose only. In the interest of efficiency, they ought to serve double duty.

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