Snapshot: Consider two snapshots of the network at different points in time. For each k, consider the set of all individuals who are k-exposed in the ﬁrst snapshot. Let ps(k) be the fraction of individuals in this set who have become adopters by the time of the second snapshot.
Figure 2 shows the shape of influence curves for the snapshot definitions using the Wikipedia data:
As with ordinal time, it is now useful to deﬁne as the ratio of two quantities, , where is the number of triples (u;C; k) for which u was k-exposed to C at time t1, and is the number of triples (u,C,k) for which u was k-exposed to C at time t1, and then joined C between t1 and t2.