chisq.test {ctest} R Documentation

Pearson's Chi-squared Test for Count Data

Description

`chisq.test` performs chi-squared tests on contingency tables.

Usage

```chisq.test(x, y = NULL, correct = TRUE,
p = rep(1/length(x), length(x)),
simulate.p.value = FALSE, B = 2000)
```

Arguments

 `x` a vector or matrix. `y` a vector; ignored if `x` is a matrix. `correct` a logical indicating whether to apply continuity correction when computing the test statistic. `p` a vector of probabilities of the same length of `x`. `simulate.p.value` a logical indicating whether to compute p-values by Monte Carlo simulation. `B` an integer specifying the number of replicates used in the Monte Carlo simulation.

Details

If `x` is a matrix with one row or column, or if `x` is a vector and `y` is not given, `x` is treated as a one-dimensional contingency table. In this case, the hypothesis tested is whether the population probabilities equal those in `p`, or are all equal if `p` is not given.

If `x` is a matrix with at least two rows and columns, it is taken as a two-dimensional contingency table, and hence its entries should be nonnegative integers. Otherwise, `x` and `y` must be vectors or factors of the same length; incomplete cases are removed, the objects are coerced into factor objects, and the contingency table is computed from these. Then, Pearson's chi-squared test of the null that the joint distribution of the cell counts in a 2-dimensional contingency table is the product of the row and column marginals is performed. If `simulate.p.value` is `FALSE`, the p-value is computed from the asymptotic chi-squared distribution of the test statistic; continuity correction is only used in the 2-by-2 case if `correct` is `TRUE`. Otherwise, if `simulate.p.value` is `TRUE`, the p-value is computed by Monte Carlo simulation with `B` replicates. This is done by random sampling from the set of all contingency tables with given marginals, and works only if the marginals are positive. (A C translation of the algorithm of Patefield (1981) is used.)

Value

A list with class `"htest"` containing the following components:

 `statistic` the value the chi-squared test statistic. `parameter` the degrees of freedom of the approximate chi-squared distribution of the test statistic, `NA` if the p-value is computed by Monte Carlo simulation. `p.value` the p-value for the test. `method` a character string indicating the type of test performed, and whether Monte Carlo simulation or continuity correction was used. `data.name` a character string giving the name(s) of the data. `observed` the observed counts. `expected` the expected counts under the null hypothesis. `residuals` the Pearson residuals, ```(observed - expected) / sqrt(expected)```.

References

Patefield, W. M. (1981) Algorithm AS159. An efficient method of generating r x c tables with given row and column totals. Applied Statistics 30, 91–97.

Examples

```data(InsectSprays)              # Not really a good example
chisq.test(InsectSprays\$count > 7, InsectSprays\$spray)
# Prints test summary
chisq.test(InsectSprays\$count > 7, InsectSprays\$spray)\$obs
# Counts observed
chisq.test(InsectSprays\$count > 7, InsectSprays\$spray)\$exp
# Counts expected under the null

## Effect of simulating p-values
x <- matrix(c(12, 5, 7, 7), nc = 2)
chisq.test(x)\$p.value           # 0.4233
chisq.test(x, simulate.p.value = TRUE, B = 10000)\$p.value
# around 0.29!

## Testing for population probabilities
## Case A. Tabulated data
x <- c(A = 20, B = 15, C = 25)
chisq.test(x)
chisq.test(as.table(x))         # the same
## Case B. Raw data
x <- trunc(5 * runif(100))
chisq.test(table(x))            # NOT 'chisq.test(x)'!
```

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