alpha: Cronbach's Coefficient Alpha
In wasabi1989/MiscPsycho: Miscellaneous Psychometric Analyses

Description Usage Arguments Details Value Author(s) Examples

Computes Cronbach's alpha. This reduces to KR-20 when the columns of the data matrix are dichotomous.

alpha(...)
## Default S3 method:
alpha(dat, ...)
## S3 method for class 'formula'
alpha(formula, data, na.action, subset, ...)

`formula`	an object of class `"formula"` (or one that can be coerced to that class): a symbolic description of the model to be fitted. The details of model specification are given under `Details`.
`data`	an optional data frame, list or environment (or object coercible by `as.data.frame` to a data frame) containing the variables in the model. If not found in `data`, the variables are taken from `environment(formula)`, typically the environment from which `alpha` is called.
`dat`	A data frame or matrix with item responses. Implemented only for the alpha.default method.
`na.action`	a function which indicates what should happen when the data contain `NA`s. Defaults to `getOption("na.action")`.
`subset`	an optional vector specifying a subset of observations to be used.
`...`	Not implemented

The conditional alpha is accesible via the summary method for objects of class alpha

A list with class "alpha" containing the following components:

`alpha`	coefficient alpha
`numItems`	the number of test items used in the computation
`condAlpha`	The alpha that would be realized if the item were excluded

Harold C. Doran

set.seed(1234)
tmp <- data.frame(item1 = sample(c(0,1), 20, replace=TRUE), item2 = sample(c(0,1), 20, replace=TRUE),
item3 = sample(c(0,1), 20, replace=TRUE),item4 = sample(c(0,1), 20, replace=TRUE),item5 = sample(c(0,1), 20, replace=TRUE))

## Formula interface
fm1 <- alpha(~ item1 + item2 + item3 + item4 + item5, data = tmp)
summary(fm1)
coef(fm1)

## Default interface
fm1 <- alpha(tmp)
summary(fm1)