Students at the middle school level are expected to use appropriate tools and techniques, including mathematics and graphing, to analyze and interpret data and communicate experimental findings. Measures of central tendency (mean, median, and mode--see sidebar for definitions) are often used as descriptive statistics when students conduct experiments in which they take repeated measures of the dependent variable or when class data are pooled for analysis. These measures of central tendency are often used in reporting results and drawing conclusions from experiments. But all scientists know that a measure of central tendency is only one feature of the data. The other important feature that must be expressed is the variability of data.