The statistic at timepoint t for dyad (i,j) is equal to the
maximum of the values of actor i and j on the attribute at
timepoint t.
Construct the `attr_actors` object as a data frame where each row represents
the attribute value of actor i at timepoint t:
name: The actors' name.
time: The time when the attribute values change.
variable: The third column contains the attribute used in the
specification of the "difference" effect. The column name should correspond
to the string supplied to the variable
argument in the `difference()`
function.
Note that it is possible to omit the `attr_actors` object in the call of
difference()
and, instead, supply it in the call of remstats()
for multiple exogenous effects.