  Most searches for alien radio transmission have focused on finding
omni-directional or purposefully earth-directed beams of enduring duration.
However, most of the interesting signals so far detected have been transient
and non-repeatable in nature. These signals could very well be the first data
points in an ever-growing data base of such signals used to construct a
probabilistic argument for the existence of extraterrestrial intelligence. This
paper looks at the effect base rate bias could have on deciding which signals
to include in such an archive based upon the likely assumption that our ability
to discern natural from artificial signals will be less than perfect.
