Dear All,
I want to replace missing data using expectation maximisation (EM) in
distributions that are skewed. SPSS EM has a function ('Mixed Normal') for
imputing missing values for skewed distributions, but to use this I need to
specify the "mixture proportion" and "standard deviation ratio". Does
anyone know how to calculate the mixture proportion and standard deviation
ratio?
My fallback option was to transform the skewed distributions and then use
EM for normally distributed data. However one of the distributions is very
positively (left) skewed and about 60% of the scores are zero. I have tried
weighting the zero scores, but I can't seem to get the distribution
anywhere near normal despite using various combinations of transformations.
Does anyone have any suggestions, either for calculating the mixture
proportion and standard deviation ratio, or for normalising the extremely
skewed distribution?
Thanks for any advice.
Regards,
John.
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