ybukhman
commented
9 years ago Sigmoid functions used by GCAT are not 0 at t=0. Therefore, b is not the same as estimated inoculation OD. b is not strictly necessary, but it does not hurt to have it. The laws of population dynamics that the functions are supposed to represent do not have b. However, GCAT's sigmoid curves are purely empirical anyway, since they fit ln(OD) instead of OD.
I would expect that log(OD-blank+delta) at inoculation should be the same as b. However, that is not the case. Looking at the formulas, it does not look like Logistic, Gompertz or Richards functions should be 0 at time 0. I have always assumed that, in the forms represented in the Zwietering's paper, they modeled log(N/N0) as a function of t. Then b = log(OD0-blank+delta), which should be the same as "corrected log OD" at inoculation. However, that does not appear to be the case. Perhaps Zwietering's formulas model log(N), rather than log(N/N0)? If that is the case, then perhaps b is not needed? Need to look further into the literature on these models and their formulas to see if they are supposed be 0 at time 0. If that is not the case, try fitting without parameter b.