Covering indexes allow data storage within the index itself while sparse indexes exclude documents that do not contain the missing field.

Covered索引允许在索引本身里存储数据，而Sparse索引则会排除掉不包含索引字段的文档。

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Sparse indexes boost performance when searching by a field that is often missing within a collection.

Sparse索引在所检索的字段并非经常出现在集合中时能提升性能。

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In the data mining or machine learning field, a data preprocessing procedure is often needed to eliminate errors, noises, in-consistent data or missing data that are contained in the dataset.

在数据挖掘以及机器学习等领域，都需要涉及一个数据预处理过程，以消除数据中所包含的错误、噪声、不一致数据或缺失值。

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If you just compare the pattern between a typical control and Mike, you can see that Mike is missing a lot of the responses in the very centre of the visual field.

如果将控制组和迈克的模型相比较，大家能够看出,迈克的视野中心缺少许多反应。

斯坦福公开课 - 7个颠覆你思想的演讲课程节选

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