ABSTRACT
The means and variances of groups capture key features of central tendency and spread, and have nice properties under suitable conditions. This chapter reviews this material as a prelude to formal comparison of groups. It begins with a graphical comparison of groups. Graphical summaries in the form of histograms, stem-and-leaf plots and box-plots provide useful information about the center, spread and other features of each group. The stem-and-leaf plot is constructed by using the most significant digit of each response as a 'stem' and accumulating the next digit as a 'leaf on the stem. Typically this involves splitting a leaf into two halves or into five parts. Box-plots reduce the distribution of data to a few key features. Box-plots capture the central shape, including skewness, as well as some features of the 'tails' of the distribution. Theory suggests that the best chance to detect differences among treatment groups occurs when the sample sizes are equal or nearly equal.
