Space-Time Cluster Analysis of Invasive Meningococcal Disease
Figure 1. The concept of space-time nearest-neighborship. Nearest-neighbors in space-time are defined as cases that are nearest-neighbors in both space and time. To define the kth nearest-neighbors in space-time, we chose the number n (e.g., n = 7; thus 7 cases [O] occurring within 15 km and 7 cases [+] occurring within 13 days) in each of the neighborhoods so that the number of cases occurring in the intersection of the two neighborhoods (⊕) equals exactly k (e.g., k = 2, the first and second nearest-neighbor in space-time). The radius is shown by the data, given a certain n. For a fixed chosen value of k, the value of n varies among cases and is found with a computer-intensive search algorithm. An example is shown of the two space-time nearest-neighbors of a given index case, by taking n = 7 at a radius of 15 km (in space), and 13 days (in time). The order is determined by increasing or decreasing the space-time intersection.