Hi Everyone,
I have a more general question about MZ and DZ correlations and their relationship with the Chronbach's alpha of the questionnaire that is used.
I am running different twin models to estimates A,C and E contribution to variation in behaviours using a parent measured questionnaire in a sample of infants and toddlers.
When running the analyses I am consistently finding large MZ correlations (0.80-0.95) for the different traits, which exceed the chronbach's alpas of the scale.
To my knowledge the Chronbach's alpha provide an upper limit to the ICCs and hence the ICCs should not be higher than the alpha.
All traits are normally distributed and substantial differences between MZ and DZ pairs are being found, so the fact that parents just rate their twins the same due to the young age of sample cannot explain this completely.
Basically: If the MZ correlation is consistently higher than the Cronbach’s alpha for many of the psychometric measures, does this mean there is systematic bias in the data?
What do you think?
Thank you for your help
M