GUTS-3S is a user-friendly stand-alone software covering all necessary steps for its use in risk assessment, i.e., as a tier 2 tool according to the aquatic guidance document. The program is part of the ring test.
Analysis. The implementation can be used for three steps of analysis:
Both reduced cases GUTS-RED-IT and GUTS-RED-SD are included. The software can handle constant as well as time-variable exposure. Exposure data can be provided in time steps down to one hour. The user may decide between exponential as well as linear interpolation of external concentration data. For calibration, several objective functions are implemented including minimizing least squares and maximizing log likelihood. The program offers the possibility to propose estimated initial values for the parameters that have to be calibrated. Furthermore, the program solves the model numerically or exactly (only possible for constant exposure). For calibration and verification, several goodness of fit measurements are included amongst others the scaled total error or model efficiency. The model results are presented visually in figures (external and internal concentration, hazard and survival over time, jpg format) and in a written report (txt format). In addition, the program includes an LC50 calculator for any duration of constant exposure. The confidence intervals of fitted values are calculated via profiling of the likelihood.
User interface and friendliness. GUTS-3S is implemented in VB.Net (Version 4.7.02053) as a user-friendly stand-alone program. The graphical user interface is intuitive. The user does not need programming skills.
Numerical methods implemented. The approximate solution of the odes of the guts cases is based on discretization using a modified Euler method. Therefore, the maximum step size is equal to one hour. In case that the relative change in a state variable (e.g., scaled damage, cumulated hazard) in one time step, or the local error, is greater than a certain bound (maximum of 0.001 is recommended), the step size is successively divided in half. The calibration of the model parameters is a non-linear optimization problem. For this, VB.Net provides the free Nelder-Mead Solver (Microsoft Solver Foundation).