alpha: Cronbach's alpha

In epiDisplay: Epidemiological Data Display Package

Cronbach's alpha

Description

Calculate reliability coefficient of items in a data frame

Usage

alpha (vars, dataFrame, casewise = FALSE, reverse = TRUE, 
    decimal = 4, vars.to.reverse = NULL, var.labels = TRUE, 
    var.labels.trunc =150)   
alphaBest (vars, dataFrame, standardized = FALSE) 

Arguments

vars	a vector containing at least three variables from the data frame
dataFrame	data frame where items are set as variables
casewise	whether only records with complete data will be used
reverse	whether item(s) negatively correlated with other majority will be reversed prior to computation
decimal	number of decimal places displayed
var.labels	presence of descriptions of variables in the last column of the output
var.labels.trunc	number of characters used for variable descriptions, long labels can be truncated
vars.to.reverse	variable(s) to reverse prior to computation
standardized	whether choosing the best subset of items is based on the standardized alpha coefficient, if FALSE then the unstandardized alpha coefficient is used

Details

This function is based on the 'reliability' function from package 'Rcmdr', which computes Cronbach's alpha for a composite scale.

There must be at least three items in 'vars' specified by their names or their index in the data frame.

The argument 'reverse' (default = TRUE) automatically reverses items negatively correlated with other majority into negative and reports the activities in the first column of the last result section. This can be overwritten by the argument 'vars.to.reverse'

Similar to the 'reliability' function, users can see the effect of removing each item on the coefficents and the item-rest correlation.

'alphaBest' is a variant of 'alpha' for successive removal of items aiming to reach the highest possible Cronbach alpha. The resultant values include variable indices of excluded and remaining items, which can be forwarded to 'tableStack' to achieve total and mean scores of the best selected items. However, there is no promise that this will give the highest possible alpha. Manual attemps may also be useful in making comparison.

Value

A list.

'alpha' returns an object of class "alpha"

alpha	unstandardized alpha coefficient
std.alpha	standardized alpha coefficient
sample.size	sample size
use.method	method for handling missing values
rbar	the average inter-item correlation
items.selected	names of variables included in the function
alpha.if.removed	a matrix of unstandardized and standardized alpha coefficients and correlation of each item with the rest of the items
result	as above but includes a column showing the items that were reversed (if TRUE) and a column of item description. As a matrix, it could be sent to a spreadsheet software using 'write.csv'
decimal	decimal places
item.labels	a character vector containing descriptions of the items

'apha.Best' returns a list of the following elements

best.alpha	the possible highest alpha obtained from the function
removed	indices of items removed by the function
remaining	indices of the remaining items
items.reversed	names of items reversed

Author(s)

Virasakdi Chongsuvivatwong [e-mail]

See Also

'cronbach' from 'psy' package and 'reliability' from 'Rcmdr' package and 'tableStack' and 'unclassDataframe' of Epicalc

Examples

data(Cars93, package="MASS")
.data <- Cars93
attach(.data)
alpha(vars=c(Min.Price:MPG.highway, EngineSize), .data)
detach(.data)

data(Attitudes)
.data <-Attitudes
attach(.data)
alpha(qa1:qa18, .data)  # Needs full screen of Rconsole
alpha(qa1:qa18, var.labels.trunc=30, .data) 
                 # Fits in with default R console screen

alpha(qa1:qa18, reverse=FALSE, .data)

alphaBest(qa1:qa18, .data) -> best.alpha
best.alpha # .7621
tableStack(best.alpha$remaining, dataFrame=.data, reverse=TRUE)

# Manual attempts by trial and error give the following
alpha(c(qa1:qa9, qa15,qa18), .data) # .7644
detach(.data)
rm(list=ls())

epiDisplay documentation built on May 18, 2022, 5:11 p.m.