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New verses - Existence Ratio

'Existence Ratio' verses configure Mona to find segments, in which there is anomalous behavior in a field’s existence ratio (the rate of contexts it appears in divided by the total number of contexts in the segment).

This set of verses currently includes “ExistenceRatioDrift” and “ExistenceRatioSuddenChange”, while “ExistenceRatioOutlier” should be released in the coming 2-3 weeks as well.

This verse is useful, for example, in cases when dealing with data sources you don’t have control of, so it might be useful to track the rate at which a field is actually populated.

Setting these verses is done in the same way as “AverageDrift” and “AverageSuddenChange” as they are based on a similar algorithm.

NOTE: Using these verse types is equivalent to creating new fields using the "existence_check" field-building function and running "Average" typed verses on them.

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