SOM Toolbox Online documentation http://www.cis.hut.fi/projects/somtoolbox/

som_gapindex

[t,r,Cd,S] = som_gapindex(sM, base, between)

 SOM_GAPINDEX Gap clustering evaluation index.

 [t,r] = som_gapindex(sM, base, [between])

  Input and output arguments ([]'s are optional):  
    sM        (struct) map struct
    base      (vector) clusters indeces for each map unit, map units
                       with index<=0 or NaN are not taken into account
    [between] (vector) indices of prototypes which are "between" clusters:
                       the associated distances are doubled
 
    t         (scalar) Gap index index for the clustering (=mean(r))
    r         (vector) maximum Gap index for each cluster (size max(base) x 1)    
 
 See also  KMEANS, KMEANS_CLUSTERS, SOM_GAPINDEX.



[ SOM Toolbox online doc ]