R is GNU S, a system for statistical computation and graphics. It consists of a language plus a run-time environment with graphics, a debugger, access to certain system functions, and the ability to run programs stored in script files. The core of R is an interpreted computer language which allows branching and looping as well as modular programming using functions. The R distribution contains functionality for a large number of statistical procedures such as: linear and generalized linear models, nonlinear regression models, time series analysis, classical parametric and nonparametric tests, clustering, smoothing and graphical data representations. Additional modules ("add-on packages") are available for a variety of specific purposes but are not distributed with this package.