以达到扩大检索范围，提高搜索引擎的查全率的目的。
That can enlarge the retrieval area and advance the recall of Searching Engine.
Google scholar里可以选择检索的年份以缩小检索范围。
Google scholar gives you an option to narrow your search to a range of years.
如果你将检索范围限制在最近1年、2年或5年，就很可能获得最新的文献，查到关于该主题最完善的总结。
If you narrow your search to the last 1, 2, or 5 years, you are likely to get the most recent papers, which often will present the best summaries of the current understanding of the topic.
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