Figure 2.
The original npc algorithm combines non‐parametric p‐values and, for imaging applications, requires substantial amount of data storage space. Two modifications simplify the procedures: (1) the statistic tk for each partial test k is transformed into a related quantity u k that has a behavior similar to the p‐values, and (2) the combined statistic is transformed to a variable that follows approximately a normal distribution, so that spatial statistics (such as cluster extent, cluster mass, and tfce) can be computed as usual. The first simplification allows the procedure to run in a single phase, without the need to retrieve data for the empirical distribution of the partial tests. [Color figure can be viewed in the online issue, which is available at http://wileyonlinelibrary.com.]