Full context:   Not much attention has been given to the study of the spacing effect in long-term retention tests. Shaughnessy found that the spacing effect is not robust for twice-presented items after a 24-hour delay in testing. The spacing effect is present, however, for items presented four or six times and tested after a 24-hour delay. This seems like a strange result and Shaughnessy interprets it as evidence for a multi-factorial account of the spacing effect.