This section outline the analysis options, system suitability criteria options and sample suitability criteria options available for the four parameter logistic model (4PL)
Note a minimum of 5 dose points is required to fit a 4PL model.
QuBAS uses the parametarisation below, based on the log dose. Other software may use the raw dose scale. The RP will be identical, but the reported values for the parameters will be different.
and
Where y is the response and x is the log (dose), for the reference standard and the test sample respectively.
is proportional to the slope at the midpoint.
If > 0: is the left asymptote
is the right asymptote
If < 0: is the right asymptote
is the left asymptote
is the log (dose) corresponding to the midpoint between and
ε is an error term with distribution N(0,1) so σi ε is the error with distribution N(0, σi2), for i = Ref or Test.
Note a minimum of 5 dose points is required to fit a 4PL model. 
is proportional to the slope at the midpoint. 
is the left asymptote
is the right asymptote
is the left asymptote
is the log (dose) corresponding to the midpoint between 