Compute variance from
    :param X:
    :return:

def sparse_var(X):
    '''
    Compute variance from
    :param X:
    :return:
    '''
    Xc = X.copy()
    Xc.data **= 2
    return np.array(Xc.mean(axis=0) - np.power(X.mean(axis=0), 2))[0]