The workhorse of the `drc` is function `drm` (Dose-Response Model) and it works pretty similarly to `lm` which we used to fit linear model. The only new thing is `fct` argument. `fct` defines the exact function to be used and some sane initial values for parameters. For four parameter logistic regression, we need to set `fct = LL.4` (log-logistic with 4 parameters, the extra "log" is just to denote that x-axis is in log scale; there is also LL.3 for instance, this sets slope to be 1).

The workhorse of the `drc` is function `drm` (Dose-Response Model) and it works pretty similarly to `lm` which we used to fit linear model. The only new thing is `fct` argument. `fct` defines the exact function to be used and some sane initial values for parameters (contrary to linear regression, estimation of parameters in non-linear regression requires some starting estimates of parameter values). For four parameter logistic regression, we need to set `fct = LL.4` (log-logistic with 4 parameters, the extra "log" is just to denote that x-axis is in log scale; e.g. there is also LL.3, three parameter version, this sets slope to be 1).

```{r}

mod = drm(rootl ~ conc, data = ryegrass, fct = LL.4())