The 4 parameter logistic model has several advantages over other curve fitting routines, but it has an inherent weakness: it is a symmetrical function and most immunoassay and bioassay data are not symmetrical. The 5 Parameter Logistic model has the flexibility to fit asymmetrical data. The 4PL, by design, makes one half of the curve exactly symmetrical to the other half. The 5PL allows each half of the curve to be different.