  The average avalanche size can be calculated exactly in a number of models of
self-organised criticality (SOC). While the calculation is straight-forward in
one dimension, it is more involved in higher dimensions and further complicated
by the presence of different boundary conditions and different forms of
external driving. Amplitudes of the leading order are determined analytically
and evaluated to obtain analytical references for numerical work. A subtle link
exists between the procedure to calculate the average avalanche size and the
field theory of SOC.
