Initial parameter dialog 
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Estimation of nonlinear regression parameters using the LevenbergMarquardt Method requires the user to supply initial guesses for each parameter. Growth II will attempt to identify useful initial guesses and these will be presented in the initial values column.
Generally you should initially see if these values lead to a solution by clicking OK. If a solution is not obtained, as can easily happen with the Richards curve, you will need to try some different guess for the initial values. These can be input by leftclicking on the existing value and typing in a new figure. One way to make some suitable guesses is to run some of the other models and look at the parameter estimates produced. For example, if the Richards curve fit fails, run a Gompertz or logistic and look at the value of the asymptotes and try these as initial guesses.
