Re: st: Why many things have Normal distribution

["Seed, Paul" <paul.seed@kcl.ac.uk>]

Coming late to the party, can I point out that 
the main alternative in nature (and econometrics) 
to a Normal distribution is a logNormal distribution.

This is so particularly for concentrations of biomarkers 
in blood, saliva, urine etc.  It tends to apply whenever 
there is a clear lower bound at zero and no upper bound; 
but can also be explained by the rules of quantitative chemistry
and the central limit theorem.

The amount of the product from any reaction is in proportion to
the products of the concentrations of the reactants.
To put it another way, it is a matter of adding the logs.  
Hence, we expect the logs to be Normally distributed.

In the case of incomes, just remember that most wage increase are 
a % of the current wage, not a fixed number of £,$ etc. per week.

BW






Paul T Seed, Senior Lecturer in Medical Statistics, 
Division of Women's Health, King's College London
Women's Health Academic Centre, King's Health Partners 
(+44) (0) 20 7188 3642.



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