seatsVotes {pscl} | R Documentation

## A class for creating seats-votes curves

### Description

Convert a vector of vote shares into a seats-vote curve object, providing estimates of partisan bias.

### Usage

    seatsVotes(x, desc = NULL, method = "uniformSwing")

### Arguments

`x` | a vector of vote shares for a specific party (either proportions or percentages)
`desc` | descriptive text
`method` | how to simulate a seats-vote curve; the only supported method at this stage is `uniformSwing`.

### Details

Simulation methods are required to induce a seats-votes curve given a vector of vote shares from one election. The uniform swing method simply slides the empirical distribution function of the vote shares “up” and “down”, computing the proportion of the vote shares that lie above .5 (by construction, the winning percentage in a two-party election) for each new location of the vector of vote shares. That is, as the empirical CDF of the observed vote shares slides up or down, more or less seats cross the .5 threshold. A seats-votes curve is formed by plotting the seat share above .5 as a function of the average district-level vote share (a weakly monotone function, since the empirical CDF constitutes a set of sufficient statistics for this problem). The simulation is run so as to ensure that average district-level vote shares range between 0 and 1.

The extent to which the seats-votes curve departs from symmetry is known as bias. More specifically, the vertical displacement of the seats-votes curve from .5 when average district-level vote share is .5 is conventionally reported as an estimate of the bias of the electoral system.

Different methods produce different estimates of seats-votes curves and summary estimands such as bias. The uniform swing method is completely deterministic and does not produce any uncertainty assessment (e.g., confidence intervals etc).

### Value

An object of class `seatsVotes`, with components

`s` | Estimated seat shares over the range of simulated average, district-level vote shares
`v` | Simulated average district-level vote shares
`x` | observed seat shares, with missing data removed
`desc` | user-supplied descriptive character string
`call` | a list of class `call`, the call to the function

### Note

Additional methods to come later.

### Author(s)

Simon Jackman simon.jackman@sydney.edu.au

### References

Tufte, Edward R. 1973. The Relationship Between Seats and Votes in Two-Party Systems. American Political Science Review. 67(2):540-554.

Gelman, Andrew and Gary King. 1990. Estimating the Consequences of Electoral Redistrictring. Journal of the American Statistical Association. 85:274-282.

Jackman, Simon. 1994. Measuring Electoral Bias: Australia, 1949-93. British Journal of Political Science. 24(3):319-357.

### See Also

`plot.seatsVotes` for plotting methods.

### Examples

    data(ca2006)
    x <- ca2006$D/(ca2006$D+ca2006$R)
    sv <- seatsVotes(x,
                     desc="Democratic Vote Shares, California 2006 congressional elections")

[Package pscl version 1.5.2 Index]