calc_trackInd: Calculate the index of the first observation of each track based on an ID variable
Description
Function to conveniently calculate the trackInd variable that is needed internally when fitting a model to longitudinal data with multiple tracks.
Usage
calc_trackInd(ID)
Value
A vector of indices of the first observation of each track which can be passed to the forward and forward_g to sum likelihood contributions of each track
Arguments
ID
ID variable of track IDs that is of the same length as the data to be analysed