  Memory Bound Functions have been proposed for fighting spam, resisting Sybil
attacks and other purposes. A particular implementation of such functions has
been proposed in which the average effort required to generate a proof of
effort is set by parameters E and l to E * l. The distribution of effort
required to generate an individual proof about this average is fairly broad.
When particular uses of these functions are envisaged, the choice of E and l,
and the system design surrounding the generation and verification of proofs of
effort, need to take the breadth of the distribution into account.
  We show the distribution for this implementation, discuss the system design
issues in the context of two proposed applications, and suggest an improved
implementation.
