We are trying to establish an age-related Normal Reference range for a
parameter from medical imaging. This measurement varies with age and
another parameter, lets call it V1. If we consider v1 then the
relationship seems linear for a part of the age range (after taking logs!)
but the variability increases in the older groups. I think the term is
heteroscedascity. We have fitted a loess function to this and it mimics
the apparent relationship well. We would like to establish reference
intervals (possibly with anon-parametric approach) and the data set is
probably large enough – Any ideas when using locally weighted regression?
I would be grateful for any advice, references, papers, software or
techniques that the list is aware of? Many thanks.
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