Thank you all for the helping answers.
I had the problem of obtaining negative Alphas, when some subjects where
excluded from analyses (three out of ten). When they were included, I
had alphas of .65 to .75 (N items =60). The problem is - as I suspect -
that the average interitem correlation is very low and drops below zero
when these subjects were excluded.
It might interest you, that I'm used to negative correlations in the
correlation matrix because I work with difference scores of reation time
measures (so there is no directional coding problem). Lots of repeated
measures ensure high consistency despite low average inter item
correlations and despite some negative correlations between individual
measures.
Nico
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