def compound_variance(data):
return np.var(np.sum(data, axis=<NUM_LIT:0>))<EOL>
Compute the variance of the compound/sum signal.
Data is first summed across units, then the variance across time
is calculated.


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


Returns
-------  
float
    variance across time of compound/sum signal


Examples
--------
>>> compound_variance(np.array([[1, 2, 3], [4, 5, 6]]))
2.6666666666666665