While the explanation for the lack of data on the number of terrorists is reasonably straight- forward, and a need to remedy this situation is pressing, the solution to the problem is less apparent. [...] The graphs consist of a set of nodes, representing people (i.e., the members of the hidden population of inter- est), and a set of edges or arcs that represent the links1 (i.e., social or spatial relationships) be- tween the nodes (St. [...] In this instance, respondents are asked to list the people they know in the population under study, and then the researcher uses a randomized device to select the people to be included in the next wave of the sample. [...] The net- work scale-up method (NSUM), on the other hand, is used to estimate the personal network size of the members of a random sample of a total population, and, using this information, to then estimate the number of members of a hidden subpopulation of the total population (Bernard et al. [...] To estimate from the social network to the population, a reciprocal model is used to calculate the number of cross-group ties between the groups in the population (Salganik and Heckathorn 2004).