shift.var(data, id.vars = NULL, shift.var = NULL, value.vars = NULL,
shift.value = -1L)
id.vars
are used to identify unique subjects,
for which shifting is done separately; e.g. with a panel data where region
refers to different regions that
all have their own time serivalue.vars
are shifted; e.g. id.vars = "year"
means shifting forward or backward in years (given one has a var name "year"
)shift.value = -1L
shifts
one row backwards (a lag of one row) and shift.value = 2L
creates a two-row lead