1. Descriptive Statistics

Statistics is the science and art of describing data and drawing inferences from them. In summarizing data for statistical purposes, our focus is usually on some characteristic that varies in a population. We refer to such characteristics as variables; height and weight are variables in a human population. Certain parameters are used to summarize such variables. There are measures of central location—principally the mean, median, and mode—that describe in various ways the center of the data; these are discussed in Section 1.2. There are measures of variability or dispersion—most notably the variance and standard deviation—that describe how widely data vary around their central value; these are described in Section 1.3. Finally, there are measures of correlation—particularly the Pearson product-moment correlation coefficient—that describe the extent to which pairs of characteristics (such as height and weight) are related in members of the population; this coefficient is described for measured data in Section 1.4. Methods for expressing association in binary data (i.e., data that can take only two values, such as 1 or 0) are described in Section 1.5.