Many statistical procedures are based on the assumption that data are normally distributed, and therefore, symmetrically distributed. However, the distributions of environmental chemical concentrations in blood or urine are often skewed. A potential reason may be that only a few people in the population were exposed to high levels of the chemical of interest while most people had a low or no exposure to the chemical. Therefore, it is important to check environmental chemical data for symmetry. The figure below presents a hypothetical presentation of these distributions. See Module 12 on Transformations for further discussion on this issue.
Figure 1. Comparison of the Normal Distribution and the Skewed Distribution in Environmental Chemical Data