Spaced repetition with expanding intervals has long been argued to be the most beneficial version of this learning procedure, but current research, which compared repetition procedures, has shown the difference between expanding repetition and uniform retrieval is either very little to nonexistent. Some researchers have found cases where uniform retrieval is better than expanding. The main speculation for this range of results is that prior research has not accounted for the possibility of their results being affected by either the spacing condition or the number of successful repetitions during study periods.