## Task 3: Key Concepts about Checking for Data Symmetry

An underlying assumption in many statistical analyses is that the
distribution of the data is normal. However, as shown in the figure below,
almost all distributions of dietary data tend to be skewed. For many
dietary constituents, a large number of persons may have zero intakes and a
few people may have very large intakes.

#### Figure 1. Hypothetical distributions of dietary intake in
the population

These
distributional characteristics are especially important when estimating the
distribution of usual intakes. Skewness does not affect simple analyses such as
those described in the Basic Dietary Analysis Course. Therefore, no special
corrections are necessary to estimate means of usual intake distributions and
statistics based on those means, such as differences between population
subgroups. However, for more complex analyses, such as those described in the
Advanced Dietary Analyses course, skewness must be taken into account. For more
information on this topic, please see the Advanced Dietary Analyses course.