Cronbach's a for dichotomous responses

[Chrys]

Dear all,

Is Cronbach's a a reliable test for questionnaires with dichotomous
responses (yes/no)? Are there any other options? Any relative
references?

Thanks!

Chrys

[Ryan]

Chrys,

I believe Cronbach's alpha in SPSS can be used for dichotomous
variables.

Ryan

[Ray Koopman]

It depends on what you expect alpha to tell you. If you're looking
for confirmation that all the items are "measuring the same thing"
then forget alpha -- you want to factor the items, and you should
probably do it using tetrachoric correlations. A high alpha does
not imply that there is only one factor, and a low alpha does not
imply that there are many factors.

The proper use of alpha is as a quick and dirty estimate of a
lower bound for the reliability of the total score. (If you're
not computing a total score then you don't care about alpha.)
The interpretation is asymmetric: a high alpha means a high
reliability, but a low alpha does not mean a low reliability.
Alpha is a function of only two things: the number of items,
and the average correlation among them. An easy way to remember
this is alpha/(1 - alpha) = k * rbar/(1 - rbar),
where k = the number of items, and
rbar = (average item-item covariance)/(average item variance),
which is a kind of weighted average item-item correlation.