Dear allstat

I hope someone will be able to help me.

I have been asked to reproduce a 4 parameter curve fit for analyte
signal vs concentration.  The curve has already been defined by the
machine that produced the signals and so I have the parameter values
that define the curve but do not know the function that relates signal
to concentration.

The output from the machine reports that a "semi-log sigmoid fit with
tails" was used.  I have looked through the internet and deduced that
semi log indicates that one of the variables is logged.  This seems to
tie in with a plot I saw (of different data but same assay) where the
concentration was plotted in the logarithmic scale (X axis).   

My understanding in that a sigmoid curve is just one that looks like an
'S' and is a special case of the logistic function.  Is it possible that
the term sigmoid is being used as a general term and may include the
logistic function?

In SAS JMP I found a 4 parameter logistic function which was S-shaped,
although when I used it to fit the data I am getting nothing meaningful
and so am currently messing around with various parameter values to try
and get a reasonable fit.  There is also a growth curve which looks as
though it may be a possible candidate, but I haven't tried it yet.

I suppose there are few questions that I have:

Does anyone know what the function would be for a "semi-log sigmoid fit
with tails" to model signal as a function of log concentration?

Is it reasonable to use an alternative 4 parameter equation even though
it is not the same as the one already evaluated?

Is there only one form of the logistic 4 parameter curve fit?

Any help would be gratefully appreciated.

Regards,
        Jayne