Fixed
A two-sided linear formula of the form f1+...+fn ~ x1+...+xm, or a list of two-sided formulas of the form f1~x1+...+xm, with possibly different models for each fixed effect (parameter) in the model. The f1,...,fn represent fixed effects included on the right hand side of model and x1+...+xm define a linear model for these parameters (when the left hand side of the formula contains several parameters, they all are assumed to follow the same linear model, described by the right hand side expression). A 1 on the right hand side of the formula(s) indicates a single fixed effects for the corresponding parameter(s). For example:
Asym + xmid + scal ~ 1