Weibull model for bacterial decay

[#1]

I have been asked to do a linear regression using Weibull on jmp. It only seems to work for data with a positive curve (growth), however, when trying to select the Weibull for the decay data it cannot be selected. Is there a parameter I am missing or an assumption that doesn't allow me to use this model?

[#2]

I will see what he says. He’s had a previous paper that compared different models… but cannot remember how he did it.

My experience with JMP and SAS is really rudimentary so thank you!