def _single_clustering(Nu, Nc):
    """Compute clustering index
    
    Parameters
    ----------
    Nu: int
        Number of units
    Nc: int
        Number of clusters
        
    Returns
    -------
    clust: float
        0 if units are not clustered (checkerboard)
        1 if units form a single cluster
    """
    clust = 1 - ( ((Nc/Nu) - (1/Nu)) /
                  (1 - (1/Nu)) ) 

    return clust