*Page updated on 11 Februari 1996*

Here are some questions and and answers about using DEBtox in practice.

**I often encounter runtime errors especially when running the profile likelihood procedure to determine the confidence parameters.**

We also have encountered this problem once or twice. Numerical procedures, especially with the reproduction experiment, are quite complex. We are still working on them to ameliorate them. Could you send me a dataset that caused a run-time error??

**The fitted parameters are highly correlated, sometimes reaching correlation coefficient equal to 1. From my experience with regression this is not acceptable and lead to errors in parameters.**

This is mostly the case when the resulting NEC is very small (practically zero), together with a very small elimination rate and very small tolerance concentration. I should expect the procedure to choose for the limit case where the elimination rate is zero.

**The NEC value is sometimes very close to 0 in contradiction with the corresponding concentration profile.**

The NEC is the no-effect concentration after infinite exposure. In the concentration profile you see the no-effect concentration after seven days. Especially when the elimination rate is very small, differences can be large. In these cases you will probably find broad confidence intervals.

**Do you know the right growth parameters for Ceriodaphnia dubia? I used the same size that you use for Daphnia magna since it is scaled and both animals get the same shape. I found 0.15 to fit well as a von Bertalanffy growth rate after some trials with different values.**

We don't know the growth parameters for Ceriodaphnia dubia. But I saw a different solution. Is it possible that you didn't start with neonates? I got a equally good fit with a growth parameter at the value 0.10 and a initial length value of 0.25.