Encoding missing values

[Bill Skaggs]

There is at least one kind of network that has no problem (in
principle) with missing inputs, namely a Boltzmann machine.
You just refrain from clamping the input node whose value is
missing, and treat it like an output node or hidden unit.

This may seem to be irrelevant to anything other than Boltzmann
machines, but I think it could be argued that nothing very much
simpler is capable of dealing with the problem.  When you ask
a network to handle missing inputs, you are in effect asking it
to do pattern completion on the input layer, and for this a
Boltzmann machine or some other sort of attractor network would
seem to be required. 

	-- Bill