This article discusses statistical measures of variability in relation to measures of central tendency and levels of measurement. Three measures of variability used in healthcare research (the range, the interquartile range, and the standard deviation) are described and compared, including their uses and limitations. The article describes how each of the three measures is calculated, and it provides a step-by-step example of calculating the sums of squares, variance, and standard deviation. Graphs of frequency and percentage distributions are used to show how the interquartile range and the standard deviation represent the variability observed within distributions. The article discusses the properties of the normal curve regarding the distribution of scores around the mean in relation to the standard deviation, and illustrates differences in the shapes of normal curves with the same mean but different standard deviations.