help alpha dialog: alpha
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Title
[R] alpha -- Compute interitem correlations (covariances) and Cronbach's
alpha
Syntax
alpha varlist [if] [in] [, options]
options description
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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
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by is allowed; see [D] by.
Menu
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
stored 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
Saved results
alpha saves 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 saved as row matrices for
the k subscales when one variable is removed.
Also see
Manual: [R] alpha
Help: [MV] factor
