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Network based statistics (NBS)
NBS (Zalesky et al., 2010) is a nonparametric statistical test used to isolate the components of an N x N undirected connectivity matrix that differ significantly between two distinct populations. Each element of the connectivity matrix stores a connectivity value and each member of the two populations possesses a distinct connectivity matrix. A component of a connectivity matrix is defined as a set of interconnected edges. The NBS comprises fours steps:
- Perform a two-sample T-test at each edge indepedently to test the hypothesis that the value of connectivity between the two populations come from distributions with equal means.
- Threshold the T-statistic available at each edge to form a set of suprathreshold edges.
- Identify any components in the adjacency matrix defined by the set of suprathreshold edges. These are referred to as observed components. Compute the size of each observed component identified; that is, the number of edges it comprises.
- Repeat K times steps 1-3, each time randomly permuting members of the two populations and storing the size of the largest component identified for each permuation. This yields an empirical estimate of the null distribution of maximal component size. A corrected p-value for each observed component is then calculated using this null distribution.
Further analysis could implement NBS predict