  We consider interstellar scintillations as a cause of intermittency in radio
signals from extraterrestrial intelligence (ETI). We demonstrate that
scintillations are very likely to allow initial detections of narrowband
signals from distant sources (> 100 pc), while making redetections improbable.
We consider three models in order to assess the non-repeating, narrowband
events found in recent SETI and to analyze large surveys in general: (I)
Radiometer noise; (II) A population of constant Galactic sources undergoing
interstellar scintillation,; and (III) Real, transient signals (or hardware
errors) of either terrestrial or ET origin.
  We apply likelihood and Bayesian tests of the models to The Planetary
Society/Harvard META data. We find that Models II and III are both highly
preferred to Model I, but that Models II and III are about equally likely.
Ruling out Model II in favor of Model III requires many more reobservations
than were conducted in META *or* the reobservation threshold must be much lower
than was used in META. *We cannot rule out the possibility that META events are
real, intrinsically steady ETI signals.*
  We recommend that future surveys use thresholds far below the typical
false-alarm threshold to lessen the effects of intermittency. The threshold
level is best defined in terms of the recording and computational technology
that is available at a cost commensurate with other survey costs.
