Stata 15 help for alpha

[MV] alpha -- Compute interitem correlations (covariances) and Cronbach's alpha

Syntax

alpha varlist [if] [in] [, options]

options Description ------------------------------------------------------------------------- Options asis take sign of each item as is casewise delete cases with missing values detail list individual interitem correlations and covariances generate(newvar) save the generated scale in newvar item display item-test and item-rest correlations label include variable labels in output table min(#) must have at least # observations for inclusion reverse(varlist) reverse signs of these variables std standardize items in the scale to mean 0, variance 1 ------------------------------------------------------------------------- by is allowed; see [D] by.

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Statistics > Multivariate analysis > Cronbach's alpha

Description

alpha computes the interitem correlations or covariances for all pairs of variables in varlist and Cronbach's alpha statistic for the scale formed from them. At least two variables must be specified with alpha.

Options

+---------+ ----+ Options +----------------------------------------------------------

asis specifies that the sense (sign) of each item be taken as presented in the data. The default is to determine the sense empirically and reverse the scorings for any that enter negatively.

casewise specifies that cases with missing values be deleted listwise. The default is pairwise computation of covariances and correlations.

detail lists the individual interitem correlations and covariances.

generate(newvar) specifies that the scale constructed from varlist be saved in newvar. Unless asis is specified, the sense of items entering negatively is automatically reversed. If std is also specified, the scale is constructed by using standardized (mean 0, variance 1) values of the individual items. Unlike most Stata commands, generate() does not use casewise deletion. A score is created for every observation for which there is a response to at least one item (one variable in varlist is not missing). The summative score is divided by the number of items over which the sum is calculated.

item specifies that item-test and item-rest correlations and the effects of removing an item from the scale be displayed. item is valid only when more than two variables are specified in varlist.

label requests that the detailed output table be displayed in a compact format that enables the inclusion of variable labels.

min(#) specifies that only cases with at least # observations be included in the computations. casewise is a shorthand for min(k), where k is the number of variables in varlist.

reverse(varlist) specifies that the signs (directions) of the variables (items) in varlist be reversed. Any variables specified in reverse() that are not also included in alpha's varlist are ignored.

std specifies that the items in the scale be standardized (mean 0, variance 1) before summing.

Examples

Setup . webuse automiss

Obtain average interitem covariance and Cronbach's alpha . alpha price headroom rep78 trunk weight length turn displ

Obtain item-test and item-rest correlations and individual interitem correlations . alpha price headroom rep78 trunk weight length turn displ, std item detail

Stored results

alpha stores the following in r():

Scalars r(alpha) scale reliability coefficient r(k) number of items in the scale r(cov) average interitem covariance r(rho) average interitem correlation if std is specified

Matrices r(Alpha) scale reliability coefficient r(ItemTestCorr) item-test correlation r(ItemRestCorr) item-rest correlation r(MeanInterItemCov) average interitem covariance r(MeanInterItemCorr) average interitem correlation if std is specified

If the option item is specified, results are stored as row matrices for the k subscales when one variable is removed.


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