Re: Least-Squares Fitting with asymetric errors

["Erik Ø. Sørensen" <sameos at mac dot com>]

On 5. mai. 2004, at 17.05, Rodney Sparapani wrote:
> > What is the best way of allowing for asymmetric errors?  Is the way to
> > do it redefine my "fi" function in both the function (to fit) and the
> > calculation of its derivative to be a step function?
> > i.e.   if ( yi <= Yi ) use the ErrorBar+ for sigma
> >       if ( yi >  Yi ) use the ErrorBar- for sigma

I am not sure of what you mean, I think a more complete description
would be necessary. But it seems to me that here you are setting
yourself up for a discontinous problem (imagina a yi that is close to 
Yi,
such that a marginal change in parameters will push an observation over 
the edge and give rise to a discrete change in the objective as one 
sigma is replaced by the other).

If I understand what you are trying to do, the likelihood should be a 
weighted sum of the expressions with the two different sigmas, with the 
weights being determined by the probability of that regime given data 
and parameters.

Erik
--
Erik Ø. Sørensen

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