def compound_mean(data):
return np.mean(np.sum(data, axis=<NUM_LIT:0>))<EOL>
Compute the mean of the compound/sum signal.
Data is first summed across units and averaged across time.


Parameters
----------
data : numpy.ndarray
    1st axis unit, 2nd axis time


Returns
-------
float
    time-averaged compound/sum signal


Examples
--------
>>> compound_mean(np.array([[1, 2, 3], [4, 5, 6]]))
7.0