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Data Structures

Zelig uses only three of R's many data structures:

1. A variable is a one-dimensional vector of length $n$.
2. A data frame is a rectangular matrix with $n$ rows and $k$ columns. Each column represents a variable and each row an observation. Each variable may have a different class. (See Section  for a list of classes.) You may refer to specific variables from a data frame using, for example, data$variable.
3. A list is a combination of different data structures. For example, z.out contains both coefficients (a vector) and data (a data frame). Use names() to view the elements available within a list, and the $ operator to refer to an element in a list.

For a more comprehensive introduction, including ways to manipulate these data structures, please refer to Chapter .