form
|
a one sided formula of the form ~ t , or code{~ t |
g}, specifying a time covariate t and, optionally, a
grouping factor g . A covariate for this correlation structure
must be integer valued. When a grouping factor is present in
form , the correlation structure is assumed to apply only
to observations within the same grouping level; observations with
different grouping levels are assumed to be uncorrelated. Defaults to
~ 1 , which corresponds to using the order of the observations
in the data as a covariate, and no groups.
|