Specifying the parameters

Since the model formula now contains both determining variables and parameters, there has to be some mechanism for specifying which are which. But of course once the parameters have been specified the remaining variates in the model formula must be variables.

As well as specifying which are the parameters, it is also necessary in this case to specify an initial approximation for each with which to start the iterative estimation procedure.

There are two ways of specifying this information:

• If the call to nls() has a start= parameter specified, its value must be a list of named components. The names of the list specify the names of the parameters and the values specify the starting values.
• If the data is held in a data frame, the parameters may similarly be defined as a parameters attribute of the data frame.

Since our policy is generally to work with data frames as much as follows, we show the second possibility in the next example.