A density function where area under a curve represents probability. When graphing the distribution of a single variable the book discusses point graphs , histograms , quantile plots and box plots . In addition to these four methods we will also consider jitter plots , density traces and empirical distributions . Density traces are like histograms with very small box widths. A density trace is an estimate of the density function of the variable.
Empirical distributions are like percentile plots turned on their sides.
The horizontal axis is formed by the values that the variable takes on and the vertical axis is the cumulative probability. Here are examples of each plot type on the same set of 200 observations from a normal distribution. Boxplots Boxplots provide a nice way to summarize the distribution of a data set with just a few numbers. Outliers are indicated by single stars or lines beyond the whiskers. Boxplots are at their best when comparing the distribution of data between several different categories.