The exponential distribution takes on a shape similar to the one in figure 5.3. If the data are exponentially distributed and require statistical evaluations, contact the Water Quality Statistician at the laboratory. There are other distributions that are found in water quality data as well, such as the Beta, Gamma and Weibull distributions. Unfortunately these distributions may take on several different shapes when graphed, and so are not easily recognizable from their plots. Independence Many of the statistical methods used with water quality data require that the sample points in the data set are independent.
Samples are independent if the sampling events have no influence on each other.
If samples are drawn too close together in space or time they will not be independent. For example, field scientist X is entrusted with gathering storm water samples which will be used to characterize runoff characteristics for a basin. X, having limited time and a definite dislike for being in the field during storms, decides to deploy all of the samplers on next Thursday because a really big storm is expected. This will allow for more than half of the samples to be collected in one day. The problem is that these samples will all be taken at one point in time and not be independent of each other over time.