Dear Statalist

I have a dataset of responses to a questionnaire. I need to calculate Cronbach's alpha using the alpha command in STATA.

I get the right output, when I run the command on the entire data set, e.g.

alpha sgi_s1 sgi_s2 sgi_s3 sgi_s4 sgi_s5 sgi_s6 ///
sgi_a1 sgi_a2 sgi_a3 sgi_a4 sgi_a5 sgi_a6 ///
sgi_a7 sgi_a8 sgi_a9 sgi_a10 ///
sgi_i1 sgi_i2 sgi_i3 sgi_i4 sgi_i5 sgi_i6 ///
sgi_i7 sgi_i8 sgi_i9 sgi_i10 sgi_i11 sgi_i12 ///
sgi_i13 sgi_i14 sgi_i15 sgi_i16 sgi_i17 sgi_i18 ///
if event==1, std item label


When I divide the participants into three groups, I sometimes get the right output, but othertimes i only get the item-test and item-rest corr., while the interitem corr. and alpha are simply returned as ".", e.g.

alpha sgi_s1 sgi_s2 sgi_s3 sgi_s4 sgi_s5 sgi_s6 ///
sgi_a1 sgi_a2 sgi_a3 sgi_a4 sgi_a5 sgi_a6 ///
sgi_a7 sgi_a8 sgi_a9 sgi_a10 ///
sgi_i1 sgi_i2 sgi_i3 sgi_i4 sgi_i5 sgi_i6 ///
sgi_i7 sgi_i8 sgi_i9 sgi_i10 sgi_i11 sgi_i12 ///
sgi_i13 sgi_i14 sgi_i15 sgi_i16 sgi_i17 sgi_i18 ///
if event==1 & diaggr==0, std item label



The problem does not always occur within the same group, so the division into groups should be ok. The number of obs also differ in all calculations. I have run the same commands without problems in another questionnaire, so I guess the problem lies within the responses to the current questionnaire, but I cannot seem to find the problem/cause.

Can anyone explain the problem to me? And is there a solution?

Thanks alot!

Thomas