Depicting variability and uncertainty using intervals and error bars.

Variability is inherent in most biological systems due to differences among members of the population. Two types of variation are commonly observed in studies: differences among samples and the "error" in estimating a population parameter (e.g. mean) from a sample. While these concepts are fundamentally very different, the associated variation is often expressed using similar notation-an interval that represents a range of values with a lower and upper bound. In this article we discuss how common intervals are used (and misused).