The shared partners effect describes the propensity of dyads to interact
based on the number of past shared partners between them. By default, the
statistic at timepoint t for the undirected dyad (i,j) is computed as
the sum of the minimum occurrences of past undirected (i,h) and undirected
(j,h) events across all actors h.
When the unique parameter is set to TRUE, a different approach is taken.
In this case, the statistic counts the number of actors h that contribute to
the creation of a new, distinct shared partner between actors i and j.
Additionally, it is possible to specify a scaling method using the scaling
parameter.
Please note that the shared partners effect, 'sp', is exclusively defined
for undirected events.