The weights of newly-hatched amphipods (small, shrimp-like crustaceans), when plotted as a frequency curve, produce a bell-shaped (or normal) curve. Figure 1a shows a typical normal distribution with a symmetrical distribution of frequency data sorted into non-overlapping value ranges. With more data, the distribution could be described by a smooth curve. Human birth weight and the body temperature of adult humans also follow this normal distribution. Compiled class results of an experiment to measure rates of enzyme action tend to form a bell-shaped curve.
However, biological data are not always normally distributed. Figure 1b shows a skewed distribution with a tail of lower values to the right: it’s a right-skewed (or positive-skewed) distribution. Examples of this type of distribution include:
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