A one sided formula of the form ~ t
, or ~ 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 formula
, 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.